Merge pull request #10885 from SimonRichardson/error-messages

#10885

## Description of change

The following handles no model names better, by helping the user move
forward, rather than dead ending them.

As a driveby I also cleaned up the issue around whitespace.

## QA steps

```sh
$ juju add-model
ERROR juju: no model name was passed. See "juju add-model --help".

Did you mean:
 juju add-model <model name>
```

------

```sh
$ juju switch
ERROR juju: no model name was passed. See "juju switch --help".

Did you mean:
 juju switch <model name>
```

cmd/juju/commands/switch.go

@@ -11,6 +11,7 @@ import (
 	"github.com/juju/errors"
 
 	jujucmd "github.com/juju/juju/cmd"
+	"github.com/juju/juju/cmd/juju/common"
 	"github.com/juju/juju/cmd/modelcmd"
 	"github.com/juju/juju/juju/osenv"
 	"github.com/juju/juju/jujuclient"
@@ -37,8 +38,8 @@ var usageSummary = `
 Selects or identifies the current controller and model.`[1:]
 
 var usageDetails = `
-When used without an argument, the command shows the current controller 
-and its active model. 
+When used without an argument, the command shows the current controller
+and its active model.
 When a single argument without a colon is provided juju first looks for a
 controller by that name and switches to it, and if it's not found it tries
 to switch to a model within current controller. mycontroller: switches to
@@ -55,7 +56,7 @@ Examples:
     juju switch mycontroller:
     juju switch :mymodel
 
-See also: 
+See also:
     controllers
     models
     show-controller`
@@ -92,7 +93,7 @@ func (c *switchCommand) Run(ctx *cmd.Context) (resultErr error) {
 			return errors.Trace(err)
 		}
 		if currentName == "" {
-			return errors.New("no currently specified model")
+			return common.MissingModelNameError("switch")
 		}
 		fmt.Fprintf(ctx.Stdout, "%s\n", currentName)
 		return nil

cmd/juju/commands/switch_test.go

@@ -13,6 +13,7 @@ import (
 	jc "github.com/juju/testing/checkers"
 	gc "gopkg.in/check.v1"
 
+	"github.com/juju/juju/cmd/juju/common"
 	"github.com/juju/juju/cmd/modelcmd"
 	_ "github.com/juju/juju/juju"
 	"github.com/juju/juju/jujuclient"
@@ -56,7 +57,7 @@ func (s *SwitchSimpleSuite) run(c *gc.C, args ...string) (*cmd.Context, error) {
 
 func (s *SwitchSimpleSuite) TestNoArgs(c *gc.C) {
 	_, err := s.run(c)
-	c.Assert(err, gc.ErrorMatches, "no currently specified model")
+	c.Assert(err, gc.ErrorMatches, common.MissingModelNameError("switch").Error())
 }
 
 func (s *SwitchSimpleSuite) TestNoArgsCurrentController(c *gc.C) {

cmd/juju/common/errors.go

@@ -58,3 +58,15 @@ func (e *TermsRequiredError) UserErr() error {
 	return errors.Wrap(e,
 		errors.Errorf(`Declined: some terms require agreement. Try: "juju agree %s"`, terms))
 }
+
+const missingModelNameMessage = `
+juju: no model name was passed. See "juju %[1]s --help".
+
+Did you mean:
+	juju %[1]s <model name>`
+
+// MissingModelNameError returns an error stating that the model name is missing
+// and provides a better UX experience to the user.
+func MissingModelNameError(cmdName string) error {
+	return errors.Errorf(missingModelNameMessage[1:], cmdName)
+}

cmd/juju/controller/addmodel.go

@@ -66,25 +66,25 @@ type addModelCommand struct {
 const addModelHelpDoc = `
 Adding a model is typically done in order to run a specific workload.
 
-To add a model, you must specify a model name. Model names can be duplicated 
-across controllers but must be unique per user for any given controller. 
-In other words, Alice and Bob can each have their own model called "secret" but 
+To add a model, you must specify a model name. Model names can be duplicated
+across controllers but must be unique per user for any given controller.
+In other words, Alice and Bob can each have their own model called "secret" but
 Alice can have only one model called "secret" in a controller.
-Model names may only contain lowercase letters, digits and hyphens, and 
+Model names may only contain lowercase letters, digits and hyphens, and
 may not start with a hyphen.
 
 To add a model, Juju requires a credential:
 
     * if you have a default (or just one) credential defined at client
      (i.e. in credentials.yaml), then juju will use that;
-    * if you have no default (and multiple) credentials defined at the client, 
+    * if you have no default (and multiple) credentials defined at the client,
      then you must specify one using --credential;
-    * as the admin user you can omit the credential, 
+    * as the admin user you can omit the credential,
      and the credential used to bootstrap will be used.
 
-To add a credential for add-model, use one of the "juju add-credential" or 
-"juju autoload-credentials" commands. These will add credentials 
-to the Juju client, which "juju add-model" will upload to the controller 
+To add a credential for add-model, use one of the "juju add-credential" or
+"juju autoload-credentials" commands. These will add credentials
+to the Juju client, which "juju add-model" will upload to the controller
 as necessary.
 
 You may also supply model-specific configuration as well as a
@@ -94,7 +94,7 @@ are the ones used to create any future resources within the model.
 If no cloud/region is specified, then the model will be deployed to
 the same cloud/region as the controller model. If a region is specified
 without a cloud qualifier, then it is assumed to be in the same cloud
-as the controller model. 
+as the controller model.
 
 Examples:
 
@@ -124,7 +124,7 @@ func (c *addModelCommand) SetFlags(f *gnuflag.FlagSet) {
 
 func (c *addModelCommand) Init(args []string) error {
 	if len(args) == 0 {
-		return errors.New("model name is required")
+		return common.MissingModelNameError("add-model")
 	}
 	c.Name, args = args[0], args[1:]
 

cmd/juju/controller/addmodel_test.go

@@ -23,6 +23,7 @@ import (
 	"github.com/juju/juju/api/base"
 	"github.com/juju/juju/apiserver/params"
 	"github.com/juju/juju/cloud"
+	"github.com/juju/juju/cmd/juju/common"
 	"github.com/juju/juju/cmd/juju/controller"
 	"github.com/juju/juju/core/model"
 	"github.com/juju/juju/environs"
@@ -134,7 +135,7 @@ func (s *AddModelSuite) TestInit(c *gc.C) {
 		values      map[string]interface{}
 	}{
 		{
-			err: "model name is required",
+			err: common.MissingModelNameError("add-model").Error(),
 		}, {
 			args: []string{"new-model"},
 			name: "new-model",