cmd/cue/cmd: print full error messages in tools mode

This sometimes didn't happen.

Fixes #383

Change-Id: I0bd98165167fc2fe6b888fdec024f065d4fda6f7
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5964
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>

cmd/cue/cmd/root.go

@@ -16,7 +16,6 @@ package cmd
 
 import (
 	"context"
-	"fmt"
 	"io"
 	"os"
 	"strings"
@@ -122,10 +121,14 @@ func MainTest() int {
 
 // Main runs the cue tool and returns the code for passing to os.Exit.
 func Main() int {
+	cwd, _ := os.Getwd()
 	err := mainErr(context.Background(), os.Args[1:])
 	if err != nil {
 		if err != ErrPrintedError {
-			fmt.Fprintln(os.Stderr, err)
+			errors.Print(os.Stderr, err, &errors.Config{
+				Cwd:     cwd,
+				ToSlash: inTest,
+			})
 		}
 		return 1
 	}

cmd/cue/cmd/testdata/script/cmd_import.txt

@@ -2,7 +2,8 @@
 cmp stderr expect-stderr
 
 -- expect-stderr --
-command.pkg.t: reference "cli" not found
+command.pkg.t: reference "cli" not found:
+    ./task_tool.cue:6:5
 -- task_tool.cue --
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