Allow for there being no "errors" in the twitter api response.

When a successful "user" query to the twitter api is completed the json that is returned is a list.
In order to validate the response the code tests to see if it was a dictionary with an "error" key.
However passing a string as a index to a list will raise a TypeError, which was not being caught by the "except" clause.

I have added TypeError to the list of items being caught. There are of course other ways of verifying the response but I think just adding the TypeError is in keeping with what you have already done.

For reference, here is what I was seeing:

> python manage.py poll_twitter --traceback --force
Traceback (most recent call last):
  ...
  File "/.../lib/python2.7/site-packages/mezzanine/twitter/models.py", line 74, in run
    raise TwitterQueryException(tweets["errors"][0]["message"])
TypeError: list indices must be integers, not str