Remove or annotate any uses of bare except: from the codebase

[alexwlchan]

As spotted in #952.

The new version of flake8 whinges about bare except:, as well it should. For now I’ve added it to the ignores list (so we can get that PR closed) – but we should go through, work out where we can do except BaseException: instead, and where we really want a bare except: (if anywhere), we explicitly annotate it with # noqa.

[Zac-HD]

I'd just ban bare excepts outright - if we don't know what exception class we're trying to catch, we should reconsider the code. Otherwise sounds great to me 😄