Make RPM build logic fail cleanly if artifact URL 404s

Some recent problems with ODL infra exposed a poor failure behavior of the RPM build logic. Carbon artifacts were not being published to Nexus, so autorelease was triggering build-rpm job with an artifact URL that didn't actually point at an artifact. The build logic was failing with a misleading error. Should be more clear.

09:24:31 File "/w/workspace/packaging-build-rpm-master/packaging/rpm/build.py", line 365, in <module>
09:24:31 version = extract_version(args.download_url)
09:24:31 File "/w/workspace/packaging-build-rpm-master/packaging/rpm/build.py", line 82, in extract_version
09:24:31 date = datetime.datetime.strptime(odl_date, "%d %b %Y").strftime(
09:24:31 File "/usr/lib64/python2.7/_strptime.py", line 325, in _strptime
09:24:31 (data_string, format))
09:24:31 ValueError: time data '' does not match format '%d %b %Y'

Imported from Trello