Revive the UserError/UsageError distinction

Revive the UserError/UsageError distinction

UsageError was removed back in

  commit bf3d434b244c57556bec979acbc658c30eb58221
  Author: W. Trevor King <wking@drexel.edu>
  Date:   Sat Dec 12 00:31:55 2009 -0500

      Added libbe.command.base (with Command class)...

because the distinction between UsageError and UserError was unclear.
I've brought it back to satisfy a request by Christian Heinrich:

On Sun, May 01, 2011 at 02:52:13AM +0200, Christian Heinrich wrote:
> 3.) Using wrong syntax should receive better help messages.
>
> Current:
>
> "be new" -> ERROR:
> Missing required argument SUMMARY
>
> Should be:
>
> "be new" -> usage: be new [options] SUMMARY
> ...

He suggested we print the full option list as well, but I've decided
to just print the usage summary and remind the user how to get the
full help message if they want it.