Output usage hint for both unknown subcommand and unknown flag in the same way

[dgn]

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.

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[github-actions[bot]]

This PR is being marked as stale due to a long period of inactivity

[craigbox]

Bump!

[virtax]

@spf13 can you please merge this commit? We need it!