enhance kubectl run error message

pkg/kubectl/cmd/run.go

@@ -156,6 +156,9 @@ func RunRun(f cmdutil.Factory, cmdIn io.Reader, cmdOut, cmdErr io.Writer, cmd *c
 
 	// validate image name
 	imageName := cmdutil.GetFlagString(cmd, "image")
+	if imageName == "" {
+		return fmt.Errorf("--image is required")
+	}
 	validImageRef := reference.ReferenceRegexp.MatchString(imageName)
 	if !validImageRef {
 		return fmt.Errorf("Invalid image name %q: %v", imageName, reference.ErrReferenceInvalidFormat)

pkg/kubectl/cmd/run_test.go

@@ -361,6 +361,13 @@ func TestRunValidations(t *testing.T) {
 		},
 		{
 			args:        []string{"test"},
+			expectedErr: "--image is required",
+		},
+		{
+			args: []string{"test"},
+			flags: map[string]string{
+				"image": "#",
+			},
 			expectedErr: "Invalid image name",
 		},
 		{