fixes(issue knative#111) generically for all command groups

Explores all sub-commands from root and adds a RunE for all
commands that are groups with child commands and without a RunE.
The added RunE will return the correct errors when the command
is called with empty sub-command or with an unknown sub-command.
It will also print the command help message first.

docs/cmd/kn_revision.md

@@ -6,6 +6,10 @@ Revision command group
 
 Revision command group
 
+```
+kn revision [flags]
+```
+
 ### Options
 
 ```

docs/cmd/kn_service.md

@@ -6,6 +6,10 @@ Service command group
 
 Service command group
 
+```
+kn service [flags]
+```
+
 ### Options
 
 ```

pkg/kn/core/root.go

@@ -15,6 +15,7 @@
 package core
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"os"
@@ -71,11 +72,36 @@ Eventing: Manage event subscriptions and channels. Connect up event sources.`,
 	rootCmd.AddCommand(commands.NewCompletionCommand(p))
 	rootCmd.AddCommand(commands.NewVersionCommand(p))
 
+	// Deal with empty and unknown sub command groups
+	emptyAndUnknownSubcommands(rootCmd)
+
 	// For glog parse error.
 	flag.CommandLine.Parse([]string{})
 	return rootCmd
 }
 
+// emptyAndUnknownSubcommands adds a RunE to all commands that are groups to
+// deal with errors when called with empty or unknown sub command
+func emptyAndUnknownSubcommands(cmd *cobra.Command) {
+	for _, childCmd := range cmd.Commands() {
+		if childCmd.HasSubCommands() && childCmd.RunE == nil {
+			childCmd.RunE = func(aCmd *cobra.Command, args []string) error {
+				aCmd.Help()
+				fmt.Println()
+
+				if len(args) == 0 {
+					return errors.New(fmt.Sprintf("please provide a valid sub-command for \"kn %s\"", aCmd.Name()))
+				} else {
+					return errors.New(fmt.Sprintf("unknown sub-command \"%s\" for \"kn %s\"", args[0], aCmd.Name()))
+				}
+			}
+
+			// recurse to deal with child commands that are themselves command groups
+			emptyAndUnknownSubcommands(childCmd)
+		}
+	}
+}
+
 // initConfig reads in config file and ENV variables if set.
 func initConfig() {
 	if commands.CfgFile != "" {