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

- change create methods to return a tuple containing the result and
  a descriptive "failure" object (instead of throwing
  NotImplementedError)
- give informative error messages when publishing comments, likes,
  or shares without a target url for the given silo
- added unit tests for the more detailed error reporting for facebook
  and twitter publishing

ref issue snarfed/bridgy#223
ah! good point, i missed the abort=True.

i see what you mean about error message w/abort=False. i'm fine with it as is. i'd also be fine with empty strings, or brief placeholder text. you're welcome to tweak them later if you want.