Show usage and exit with error whenever arguments parsing fails

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>

app.go

@@ -2,6 +2,8 @@ package cli
 
 import (
 	"os"
+	"io/ioutil"
+	"fmt"
 )
 
 type App struct {
@@ -40,12 +42,17 @@ func (a *App) Run(arguments []string) {
 
 	// parse flags
 	set := flagSet(a.Name, a.Flags)
+	set.SetOutput(ioutil.Discard)
 	err := set.Parse(arguments[1:])
+	context := NewContext(a, set, set)
+
 	if err != nil {
+		fmt.Println("Incorrect Usage.\n")
+		ShowAppHelp(context)
+		fmt.Println("")
 		os.Exit(1)
 	}
 
-	context := NewContext(a, set, set)
 	checkHelp(context)
 	checkVersion(context)
 

command.go

@@ -1,5 +1,11 @@
 package cli
 
+import (
+	"io/ioutil"
+	"os"
+	"fmt"
+)
+
 type Command struct {
 	Name        string
 	ShortName   string
@@ -17,7 +23,15 @@ func (c Command) Run(ctx *Context) {
 	)
 
 	set := flagSet(c.Name, c.Flags)
-	set.Parse(ctx.Args()[1:])
+	set.SetOutput(ioutil.Discard)
+	err := set.Parse(ctx.Args()[1:])
+
+	if err != nil {
+		fmt.Println("Incorrect Usage.\n")
+		ShowCommandHelp(ctx, c.Name)
+		fmt.Println("")
+		os.Exit(1)
+	}
 
 	context := NewContext(ctx.App, set, ctx.globalSet)
 	checkCommandHelp(context, c.Name)