dm: cleanup CLI UX for people getting started

Before this patch dm would print out the wall of usage text that would
scroll off the screen in many error cases. This made learning dm hard.
Instead have more compact error output, like kubectl, and only return
non-zero on real errors.

```
$ dm foo ; echo $?
Error: unknown command 'foo' for 'dm'
Run 'dm --help' for usage
1
$ dm deployed-instances; echo $?
No type name supplied
1
$ dm 2> /dev/null; echo $?
0
```

There is more to do to make this tool friendlier but this was
low-hanging.

dm/dm.go

@@ -83,7 +83,7 @@ var usage = func() {
 	fmt.Fprintln(os.Stderr)
 	fmt.Fprintln(os.Stderr, "--stdin requires a file name and either the file contents or a tar archive containing the named file.")
 	fmt.Fprintln(os.Stderr, "        a tar archive may include any additional files referenced directly or indirectly by the named file.")
-	panic("\n")
+	os.Exit(0)
 }
 
 func getGithubCredential() *common.RegistryCredential {
@@ -136,7 +136,6 @@ func execute() {
 	flag.Parse()
 	args := flag.Args()
 	if len(args) < 1 {
-		fmt.Fprintln(os.Stderr, "No command supplied")
 		usage()
 	}
 
@@ -173,7 +172,7 @@ func execute() {
 	case "get":
 		if len(args) < 2 {
 			fmt.Fprintln(os.Stderr, "No deployment name supplied")
-			usage()
+			os.Exit(1)
 		}
 
 		path := fmt.Sprintf("deployments/%s", args[1])
@@ -183,14 +182,14 @@ func execute() {
 		msg := "Must specify manifest in the form <deployment>/<manifest> or <deployment> to list."
 		if len(args) < 2 {
 			fmt.Fprintln(os.Stderr, msg)
-			usage()
+			os.Exit(1)
 		}
 
 		s := strings.Split(args[1], "/")
 		ls := len(s)
 		if ls < 1 || ls > 2 {
 			fmt.Fprintln(os.Stderr, fmt.Sprintf("Invalid manifest (%s), %s", args[1], msg))
-			usage()
+			os.Exit(1)
 		}
 
 		path := fmt.Sprintf("deployments/%s/manifests", s[0])
@@ -203,7 +202,7 @@ func execute() {
 	case "delete":
 		if len(args) < 2 {
 			fmt.Fprintln(os.Stderr, "No deployment name supplied")
-			usage()
+			os.Exit(1)
 		}
 
 		path := fmt.Sprintf("deployments/%s", args[1])
@@ -219,7 +218,7 @@ func execute() {
 	case "deployed-instances":
 		if len(args) < 2 {
 			fmt.Fprintln(os.Stderr, "No type name supplied")
-			usage()
+			os.Exit(1)
 		}
 
 		tUrls := getDownloadURLs(args[1])
@@ -237,7 +236,9 @@ func execute() {
 	case "registries":
 		callService("registries", "GET", "list registries", nil)
 	default:
-		usage()
+		fmt.Fprintf(os.Stderr, "Error: unknown command '%s' for 'dm'\n", args[0])
+		fmt.Fprintln(os.Stderr, "Run 'dm --help' for usage")
+		os.Exit(1)
 	}
 }
 
@@ -292,7 +293,7 @@ func callHttp(path, method, action string, reader io.ReadCloser) string {
 func describeType(args []string) {
 	if len(args) != 2 {
 		fmt.Fprintln(os.Stderr, "No type name or URL supplied")
-		usage()
+		os.Exit(1)
 	}
 
 	tUrls := getDownloadURLs(args[1])
@@ -320,12 +321,12 @@ func loadTemplate(args []string) *common.Template {
 	var err error
 	if len(args) < 2 {
 		fmt.Fprintln(os.Stderr, "No template name or configuration(s) supplied")
-		usage()
+		os.Exit(1)
 	}
 
 	if *stdin {
 		if len(args) < 2 {
-			usage()
+			os.Exit(1)
 		}
 
 		input, err := ioutil.ReadAll(os.Stdin)