* Error reporting sucks, and when a domain or user doesn't exist we
  should be able to say so. The describe_domain/user functions
  should also work better. For plugins that don't implement domains,
  we can return a (count of?) list of users, or fall back to the
  user deletion descriptions.

* Error reporting is inconsistent. We know what goes wrong, and then
  errors bubble up, but where do they get reported? Are they fatal or
  informational? Do we want later plugins to run of earlier ones
  failed?

* Implement "mv".

  Design:

  This should only work from a domain that exists in postfixadmin to a
  domain that already exists in postfixadmin. If the target domain
  does not exist, we should error out as soon as possible.

  Once we're sure that the target domain does exist, we can hand off
  the "mv" operation to the plugins. Some of them might have nothing
  to do -- that's fine.

  MAKE SURE WE DON'T OVERWRITE AN EXISTING USER!

* Add convenience methods (e.g. user_exists, domain_exists) that we
  can use in the tests to check results.

* Potentially add OpenDKIM support.