Cleanup Hipchat error handling and return values.

1. Use the @utils.retry decorator to retry all requests when an
HTTPError is raised. Specifically we're looking at transient errors like
502/503. If a non-transient error (like a 401, 403) is raised, it will
ultimately hit the exception block and be raised appropriately.

    Closes #74, #75.

2. Stop returning False, we don't do that anymore.
3. Catch any HTTPError exception and re-wrap it as an Actor exception.
Merge pull request #130 from Nextdoor/cleanup_hipchat_actor

Cleanup Hipchat error handling and return values.