refresh_peps_urls should check the response status code

[sentry]

Sentry Issue: BOT-JD

In the case below, the GitHub rate limit was exceed and a 403 was returned.

TypeError: string indices must be integers
  File "bot/exts/utils/utils.py", line 224, in refresh_peps_urls
    name = file["name"]

Task exception was never retrieved
future: <Task finished name='Task-10' coro=<Utils.refresh_peps_urls() done, defined at /bot/bot/exts/utils/utils.py:212> exception=TypeError('string indices must be integers')>