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
Added David Higgins to AUTHORS
Merge.
Merge.
Added hanchen to AUTHORS.
Fix tag cloud factor in generic app.
Version bump to 1.4.13
Added tag 1.4.13 for changeset 90aff0815bf0
Update auto-generated changelog, requirements.
Blog: Catch exception if a non-existent month is requested from the archives.
Version bump to 1.4.14
Added tag 1.4.14 for changeset e83ad5bdfedc
Blog: Generate RSS and Atom feeds through richtext_filters.
Update auto-generated changelog, requirements.
Merge.
Delete BS2 static resources. Add BS3 resources: css, js and fonts.
Migrated base.html and all it's includes to BS3.
Strip STATIC_URL, leading / from proxied URLs

STATIC_URL often contains host or generic_host (esp. if STATIC_URL is a
path on the same domain), so it needs to be removed first to ensure it
is removed completely. Also removed leading '/' from URL, since it
appears staticfiles doesn't like absolute paths.
New sites in README
Merge.