Fixed TypeError

In some cases Twitter API returns only one error at this case response JSON is like this:

    {u'errors': u'sharing is not permissible for this status (Share validations failed)'}

But _get_error_message at api.py trying to catch 0th error message on response:

    line 216 at api.py
    error_message = content['errors'][0]['mesage']

That raises: TypeError: string indices must be integers

So i added an exception to handle this case. I'm waiting for your comments.