Consistent usage hint for unknown subcommand and flag

Now, when you try to run a Command with an unknown sub-command vs. an unknown flag, you'll get:
```
$ testcommand asdsa
Error: unknown command "asdsa" for "testcommand"
Run 'testcommand --help' for usage.

$ testcommand --asdsa
Error: unknown flag: --asdsa

$ testcommand -asdsa
Error: unknown shorthand flag: 'a' in -asdsa
```

With these changes, you'll get:
```
$ testcommand asdsa
Error: unknown command "asdsa" for "testcommand"
Run 'testcommand --help' for usage.

$ testcommand --asdsa
Error: unknown flag: --asdsa
Run 'testcommand --help' for usage.

$ testcommand -asdsa
Error: unknown shorthand flag: 'a' in -asdsa
Run 'testcommand --help' for usage.
```

Merge spf13/cobra#1023

command.go

@@ -433,6 +433,11 @@ func (c *Command) UsageString() string {
 	return bb.String()
 }
 
+// UsageHintString returns a string that describes how to obtain usage instructions
+func (c *Command) UsageHintString() string {
+	return fmt.Sprintf("Run '%v --help' for usage.\n", c.CommandPath())
+}
+
 // FlagErrorFunc returns either the function set by SetFlagErrorFunc for this
 // command or a parent, or it returns a function which returns the original
 // error.
@@ -964,7 +969,7 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 		}
 		if !c.SilenceErrors {
 			c.PrintErrln("Error:", err.Error())
-			c.PrintErrf("Run '%v --help' for usage.\n", c.CommandPath())
+			c.PrintErrf(cmd.UsageHintString())
 		}
 		return c, err
 	}
@@ -999,6 +1004,9 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 		// all subcommands should respect it
 		if !cmd.SilenceUsage && !c.SilenceUsage {
 			c.Println(cmd.UsageString())
+		} else if !cmd.SilenceErrors && !c.SilenceErrors {
+			// if SilenceUsage && !SilenceErrors, we should be consistent with the unknown sub-command case and output a hint
+			c.Printf(cmd.UsageHintString())
 		}
 	}
 	return cmd, err