[error] Better error message for ExitErrors (#675)

## Summary

This PR makes three changes:

1. Handles ExitErrors specially (for non `usererr.ExitError`) by
printing the `err.Stderr` when `DEVBOX_DEBUG=1`.

2. When `DEVBOX_DEBUG=0`, we now include the command and also suggest to
the user to run with DEVBOX_DEBUG=1 for a more detailed error and
suggest reporting the bug.

3. Renames `usererr.UserExecError` to `usererr.ExitError` which more
accurately represents what that struct is doing: the error is only
wrapped if it is ExitError.

## How was it tested?

in `testdata/unfree-packages`:


BEFORE:
```
❯ DEVBOX_FEATURE_FLAKES=1 DEVBOX_FEATURE_UNIFIED_ENV=1 DEVBOX_DEBUG=0 devbox shell
Ensuring packages are installed.
Ensuring nixpkgs registry is downloaded.
Downloaded 'github:NixOS/nixpkgs/52e3e80afff4b16ccb7c52e9f0f5220552f03d04' to '/nix/store/j5a7i3dvxslg2ychfy53wdhg1m3xfrwm-source' (hash 'sha256-FqZ7b2RpoHQ/jlG6JPcCNmG/DoUPCIvyaropUDFhF3Q=').
Ensuring nixpkgs registry is downloaded: Success
Starting a devbox shell...
Error: exit status 1
```

AFTER:

<img width="1239" alt="Screenshot 2023-02-23 at 1 21 30 PM"
src="https://user-images.githubusercontent.com/676452/221033169-92c65573-a339-422d-8141-5ba9c58e0353.png">

and AFTER with DEBUG=1:


<img width="1217" alt="Screenshot 2023-02-23 at 1 15 50 PM"
src="https://user-images.githubusercontent.com/676452/221033213-ded4c288-7fec-425e-bf7e-7a75c5323f6a.png">

internal/boxcli/midcobra/debug.go

@@ -4,8 +4,9 @@
 package midcobra
 
 import (
-	"fmt"
+	"errors"
 	"os"
+	"os/exec"
 	"strconv"
 
 	"github.com/fatih/color"
@@ -59,12 +60,15 @@ func (d *DebugMiddleware) postRun(cmd *cobra.Command, args []string, runErr erro
 		} else {
 			color.New(color.FgRed).Fprintf(cmd.ErrOrStderr(), "\nError: %s\n\n", runErr.Error())
 		}
-	} else {
-		fmt.Fprintf(cmd.ErrOrStderr(), "Error: %v\n", runErr)
 	}
 
 	st := debug.EarliestStackTrace(runErr)
-	debug.Log("Error: %v\nExecutionID:%s\n%+v\n", runErr, d.executionID, st)
+	color.New(color.FgRed).Fprintf(cmd.ErrOrStderr(), "Error: %v\n\n", runErr)
+	var exitErr *exec.ExitError
+	if errors.As(runErr, &exitErr) {
+		debug.Log("Command stderr: %s\n", exitErr.Stderr)
+	}
+	debug.Log("\nExecutionID:%s\n%+v\n", d.executionID, st)
 }
 
 func (d *DebugMiddleware) withExecutionID(execID string) Middleware {

internal/boxcli/midcobra/midcobra.go

@@ -12,6 +12,8 @@ import (
 	"github.com/google/uuid"
 	"github.com/spf13/cobra"
 	"go.jetpack.io/devbox/internal/boxcli/usererr"
+	"go.jetpack.io/devbox/internal/debug"
+	"go.jetpack.io/devbox/internal/ux"
 )
 
 type Executable interface {
@@ -64,7 +66,7 @@ func (ex *midcobraExecutable) Execute(ctx context.Context, args []string) int {
 	err := ex.cmd.ExecuteContext(ctx)
 
 	var postRunErr error
-	var userExecErr *usererr.UserExecError
+	var userExecErr *usererr.ExitError
 	// If the error is from a user exec call, exclude such error from postrun hooks.
 	if err != nil && !errors.As(err, &userExecErr) {
 		postRunErr = err
@@ -82,11 +84,16 @@ func (ex *midcobraExecutable) Execute(ctx context.Context, args []string) int {
 		// If the error is from the exec call, return the exit code of the exec call.
 		// Note: order matters! Check if it is a user exec error before a generic exit error.
 		var exitErr *exec.ExitError
-		var userExecErr *usererr.UserExecError
+		var userExecErr *usererr.ExitError
 		if errors.As(err, &userExecErr) {
 			return userExecErr.ExitCode()
 		}
 		if errors.As(err, &exitErr) {
+			if !debug.IsEnabled() {
+				ux.Ferror(ex.cmd.ErrOrStderr(), "There was an internal error. "+
+					"Run with DEVBOX_DEBUG=1 for a detailed error message, and consider reporting it at "+
+					"https://github.com/jetpack-io/devbox/issues\n")
+			}
 			return exitErr.ExitCode()
 		}
 		return 1 // Error exit code

internal/boxcli/usererr/execerr.go → internal/boxcli/usererr/exiterr.go

@@ -1,12 +1,12 @@
 package usererr
 
 import (
-	"os/exec"
-
 	"errors"
+	"os/exec"
 )
 
-type UserExecError struct {
+// ExitError is an ExitError for a command run on behalf of a user
+type ExitError struct {
 	err *exec.ExitError
 }
 
@@ -19,22 +19,22 @@ func NewExecError(source error) error {
 	if !errors.As(source, &exitErr) {
 		return source
 	}
-	return &UserExecError{
+	return &ExitError{
 		err: exitErr,
 	}
 }
 
-func (e *UserExecError) Error() string {
+func (e *ExitError) Error() string {
 	return e.err.Error()
 }
 
-func (e *UserExecError) Is(target error) bool {
+func (e *ExitError) Is(target error) bool {
 	return errors.Is(e.err, target)
 }
 
-func (e *UserExecError) ExitCode() int {
+func (e *ExitError) ExitCode() int {
 	return e.err.ExitCode()
 }
 
 // Unwrap provides compatibility for Go 1.13 error chains.
-func (e *UserExecError) Unwrap() error { return e.err }
+func (e *ExitError) Unwrap() error { return e.err }

internal/impl/packages.go

@@ -98,8 +98,7 @@ func (d *Devbox) addPackagesToProfile(mode installMode) error {
 		if err != nil {
 			fmt.Fprintf(d.writer, "%s: ", stepMsg)
 			color.New(color.FgRed).Fprintf(d.writer, "Fail\n")
-
-			return errors.New(commandErrorMessage(cmd, err))
+			return errors.Wrapf(err, "Command: %s", cmd)
 		}
 
 		fmt.Fprintf(d.writer, "%s: ", stepMsg)
@@ -240,29 +239,9 @@ func (d *Devbox) ensureNixpkgsPrefetched() error {
 	if err := cmd.Run(); err != nil {
 		fmt.Fprintf(d.writer, "Ensuring nixpkgs registry is downloaded: ")
 		color.New(color.FgRed).Fprintf(d.writer, "Fail\n")
-		return errors.New(commandErrorMessage(cmd, err))
+		return errors.Wrapf(err, "Command: %s", cmd)
 	}
 	fmt.Fprintf(d.writer, "Ensuring nixpkgs registry is downloaded: ")
 	color.New(color.FgGreen).Fprintf(d.writer, "Success\n")
 	return nil
 }
-
-// Consider moving to cobra middleware where this could be generalized. There is
-// a complication in that its current form is useful because of the exec.Cmd. This
-// would be missing in the middleware, unless we pass it along by wrapping the error in
-// another struct.
-func commandErrorMessage(cmd *exec.Cmd, err error) string {
-	var errorMsg string
-
-	// ExitErrors can give us more information so handle that specially.
-	var exitErr *exec.ExitError
-	if errors.As(err, &exitErr) {
-		errorMsg = fmt.Sprintf(
-			"Error running command %s. Exit status is %d. Command stderr: %s",
-			cmd, exitErr.ExitCode(), string(exitErr.Stderr),
-		)
-	} else {
-		errorMsg = fmt.Sprintf("Error running command %s. Error: %v", cmd, err)
-	}
-	return errorMsg
-}

internal/nix/nix.go

@@ -155,7 +155,7 @@ func PrintDevEnv(nixShellFilePath, nixFlakesFilePath string) (*varsAndFuncs, err
 	cmd.Env = DefaultEnv()
 	out, err := cmd.Output()
 	if err != nil {
-		return nil, errors.WithStack(err)
+		return nil, errors.Wrapf(err, "Command: %s", cmd)
 	}
 
 	var vaf varsAndFuncs