Exit non-zero if a unknown subcommand is given

Currently it just prints the help message and exits 0.

We do this by modifying the helpCommand and helpSubcommand cli.Commands
to return an error if they are called with an unknown subcommand. This
propogates up to the app which exits with 3 and prints the error.

Thanks to @danslimmon for the initial approach!

Fixes #276

help.go

@@ -81,10 +81,10 @@ var helpCommand = Command{
 	Action: func(c *Context) error {
 		args := c.Args()
 		if args.Present() {
-			ShowCommandHelp(c, args.First())
-		} else {
-			ShowAppHelp(c)
+			return ShowCommandHelp(c, args.First())
 		}
+
+		ShowAppHelp(c)
 		return nil
 	},
 }
@@ -97,11 +97,10 @@ var helpSubcommand = Command{
 	Action: func(c *Context) error {
 		args := c.Args()
 		if args.Present() {
-			ShowCommandHelp(c, args.First())
-		} else {
-			ShowSubcommandHelp(c)
+			return ShowCommandHelp(c, args.First())
 		}
-		return nil
+
+		return ShowSubcommandHelp(c)
 	},
 }
 
@@ -127,30 +126,31 @@ func DefaultAppComplete(c *Context) {
 }
 
 // Prints help for the given command
-func ShowCommandHelp(ctx *Context, command string) {
+func ShowCommandHelp(ctx *Context, command string) error {
 	// show the subcommand help for a command with subcommands
 	if command == "" {
 		HelpPrinter(ctx.App.Writer, SubcommandHelpTemplate, ctx.App)
-		return
+		return nil
 	}
 
 	for _, c := range ctx.App.Commands {
 		if c.HasName(command) {
 			HelpPrinter(ctx.App.Writer, CommandHelpTemplate, c)
-			return
+			return nil
 		}
 	}
 
-	if ctx.App.CommandNotFound != nil {
-		ctx.App.CommandNotFound(ctx, command)
-	} else {
-		fmt.Fprintf(ctx.App.Writer, "No help topic for '%v'\n", command)
+	if ctx.App.CommandNotFound == nil {
+		return NewExitError(fmt.Sprintf("No help topic for '%v'", command), 3)
 	}
+
+	ctx.App.CommandNotFound(ctx, command)
+	return nil
 }
 
 // Prints help for the given subcommand
-func ShowSubcommandHelp(c *Context) {
-	ShowCommandHelp(c, c.Command.Name)
+func ShowSubcommandHelp(c *Context) error {
+	return ShowCommandHelp(c, c.Command.Name)
 }
 
 // Prints the version number of the App