TwitterError standard form

I think it would be much better to raise errors that have common form: for example a dictionary with number, message and so on. Even if some of them will be empty it is still easier to parse than check what's inside that error, if it is a list of dictionary, string or something else. For example: if you try to get timeline from user that doesn't exist, it returns:
twitter.TwitterError([{u'code': 34, u'message': u'Sorry, that page does not exist'}])
but when you try to access user with private tweets it raises only that:
twitter.TwitterError(u'Not authorized.')

[bear]

Those error messages come from Twitter itself, all the exception is doing is including the response body from the Twitter call...

I'll take a look and see if we can ensure that the error is always a dictionary at least

[bear]

done - we may need to tweak things, but closing this as done for now