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.
publish: flip out of order logging parameters
remove as_source._creation... helper methods
change CreationFailure to CreationResult unifying return value
from create methods
Merge pull request #251 from kylewm/publish-reply-errors2

publish: improve error reporting for comments, likes, and shares
move requests mocking support to activitystreams-unofficial

matches snarfed/activitystreams-unofficial@1e346a8
if we send a webmention to a source's domain, also set its webmention_endpoint if unset

fixes #249
preserve newlines in log output
propagate: give up target urls on DNS failures. fixes #254
ereporter exception