Retry TextIt requests if needed.

[toolness]

Fixes #1567 using a mechanism outlined in the Celery documentation on retrying tasks.

I originally used Celery's autoretry_for keyword argument here but it was impossible to test (or even to test that I wasn't misspelling autoretry_for, as renaming the argument to LOL resulted in everything still working) so I reverted to doing things the long way around. I'm still not particularly convinced this is a good test, or that the retry logic will even work in production, but whatever.