Help handling for group commands

[emmiegit]

Right now for some command group group, if you have malformed arguments, then it'll send the help message as if you just did !group. Perhaps we should have it so that an error response is sent if !group invalid command is sent rather than sending the help message.

While we're at it, we can replace our current if ctx.invoked_subcommand ... raise SendHelp() stuff with an inherited or helper method to do all of that for us in one place.