Enable H201: do not write "except:"

Fixes all occurrences of this. We have a custom exception handler
in Horizon anyway that only catches ClientException in most of these
cases, but this commit lets us gate on the other cases.

Change-Id: Iea3dc13817f3e5b775b2024424bf3a906da5584b
Closes-Bug: #1211657
Merge "Enable H201: do not write "except:""