[#21] Improve gather stage error handling

See issue for full details. Basically we don't want to catch any
exception at the queue.py level, as they prevent debugging. Harvesters
should deal with them and return a list of ids or an empty list if no
objects need to be fetched.
Also improved the debug messages.
Merge branch 'release-v2.0' of github.com:okfn/ckanext-harvest into release-v2.0