publish: improve error reporting for comments, likes, and shares

- updated to accept activitystreams' new tuple return value for _create
  methods
- move some error-checking into activitystreams-unofficial which is
  in a better position to give detailed error messages.

ref issue #223
i see what you mean. there were a couple spots in bridgy where I had to import `activitystreams.source.CreationFailure` that did feel a little bit awkward