Fix undefined name 'URLError' and use new style exceptions

[cclauss]

flake8 testing of https://github.com/ros-infrastructure/buildfarm on Python 2.7.14

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./buildfarm/storm.py:154:16: F821 undefined name 'URLError'
        except URLError, ex:
               ^
./buildfarm/storm.py:163:16: F821 undefined name 'URLError'
        except URLError, ex:
               ^
2     F821 undefined name 'URLError'

[tfoote]

This looks reasonable. Though this code base is basically legacy. (For example we're not using storm on demand anymore which this integrates with) We should consider testing out the new github Archive mode for it.

[cclauss]

Is there a reason not to merge these changes before archiving?

[tfoote]

In general I'd rather not merge this unless it's tested. And as far as I know the API that this is hitting has likely evolved since last tested so keeping this clearly as a snapshot from a few years ago better communicates the state of the system.