cmd: refactor common err expr into helper function

The same redundant fmt.Sprintf() and string literal was duplicated
throughout many of the files in kubectl/cmd. Replace with a helper
function.

pkg/kubectl/cmd/create_serviceaccount.go

@@ -17,7 +17,6 @@ limitations under the License.
 package cmd
 
 import (
-	"fmt"
 	"io"
 
 	"github.com/spf13/cobra"
@@ -69,7 +68,7 @@ func CreateServiceAccount(f cmdutil.Factory, cmdOut io.Writer, cmd *cobra.Comman
 	case cmdutil.ServiceAccountV1GeneratorName:
 		generator = &kubectl.ServiceAccountGeneratorV1{Name: name}
 	default:
-		return cmdutil.UsageError(cmd, fmt.Sprintf("Generator: %s not supported.", generatorName))
+		return errUnsupportedGenerator(cmd, generatorName)
 	}
 	return RunCreateSubcommand(f, cmd, cmdOut, &CreateSubcommandOptions{
 		Name:                name,