Improve error reporting

This commit introduces several improvements to the way bosun reacts to
and reports errors.

Instead of doing nothing and exiting silently, alert the user and print
a short usage synopsis if bosun does not understand a command it was
passed. Also, alert the user and print available list types if the
`list` command was invoked with a type it does not understand.  Lastly,
check for the existence of a role before trying to `add`, `remove`, or
`rebuild` it.

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