Updates NoArgs to also report invalid argument

NoArgs currently reports "unknown command" in all cases, which
can be is a little misleading to the user if there are no
subcommands. Change it as follows

- if subcommands and one extra arg, say "unknown command"
- otherwise, say "invalid argument(s)" and show all extra args

This should produce more meaningful error messages to users, and
works for both the root and sub-commands.

args.go

@@ -2,6 +2,7 @@ package cobra
 
 import (
 	"fmt"
+	"strings"
 )
 
 type PositionalArgs func(cmd *Command, args []string) error
@@ -25,10 +26,18 @@ func legacyArgs(cmd *Command, args []string) error {
 
 // NoArgs returns an error if any args are included.
 func NoArgs(cmd *Command, args []string) error {
-	if len(args) > 0 {
+	if len(args) == 0 {
+		return nil
+	}
+
+	// If we have exactly one argument and we have sub-commands, then assume the user
+	// made a typo on a subcommand, or typed the wrong subcommand
+	if len(args) == 1 && len(cmd.commands) > 0 {
 		return fmt.Errorf("unknown command %q for %q", args[0], cmd.CommandPath())
 	}
-	return nil
+
+	// Otherwise, assume that this is one or more invalid arguments
+	return fmt.Errorf("invalid argument(s) %q for %q", strings.Join(args, " "), cmd.CommandPath())
 }
 
 // OnlyValidArgs returns an error if any args are not in the list of ValidArgs.

args_test.go

@@ -26,7 +26,7 @@ func TestNoArgsWithArgs(t *testing.T) {
 	}
 
 	got := err.Error()
-	expected := `unknown command "illegal" for "c"`
+	expected := `invalid argument(s) "illegal" for "c"`
 	if got != expected {
 		t.Errorf("Expected: %q, got: %q", expected, got)
 	}
@@ -223,7 +223,7 @@ func TestChildTakesNoArgs(t *testing.T) {
 	}
 
 	got := err.Error()
-	expected := `unknown command "illegal" for "root child"`
+	expected := `invalid argument(s) "illegal args" for "root child"`
 	if !strings.Contains(got, expected) {
 		t.Errorf("expected %q, got %q", expected, got)
 	}