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'm starting to wonder if this whole thing - finding candidate items, trying them in some order, short circuiting and aborting when necessary - should be inside a-u. as is, it's awkwardly straddling the API boundary right now.

not at all your fault or caused by these changes, they just may have had the (mis)fortune of pushing it over the threshold.

that'd definitely be a separate task, though, and i'm happy for this to go in as is. just thinking out loud.