Retry downloads in case it was just an OBS hick

[cbosdo]

Adding a max retry as config and defaulting to 3 or 5 would be cool.
While at it printing the URL of the failed download instead of a backtrace would help a lot understanding what happens.

Example of such backtrace:

     [exec] INFO:root:Downloading /tmp/obsmvn-y1hlpv40/ical4j-3.0.18-6.5.uyuni1.noarch.rpm
     [exec] Traceback (most recent call last):
     [exec]   File "/usr/bin/obs-to-maven", line 11, in <module>
     [exec]     load_entry_point('obs-to-maven==1.1.0', 'console_scripts', 'obs-to-maven')()
     [exec]   File "/usr/lib/python3.6/site-packages/obs_maven/core.py", line 79, in main
     [exec]     artifact.process(config.repo, tmp)
     [exec]   File "/usr/lib/python3.6/site-packages/obs_maven/artifact.py", line 223, in process
     [exec]     rpm_file = self.fetch_binary(file, tmp)
     [exec]   File "/usr/lib/python3.6/site-packages/obs_maven/artifact.py", line 63, in fetch_binary
     [exec]     self.repository.get_binary(file.path, target_file, file.mtime)
     [exec]   File "/usr/lib/python3.6/site-packages/obs_maven/repo.py", line 69, in get_binary
     [exec]     f = urllib.request.urlopen("{}/{}/{}/{}".format(self.uri, self.project, self.repository, path))
     [exec]   File "/usr/lib64/python3.6/urllib/request.py", line 223, in urlopen
     [exec]     return opener.open(url, data, timeout)
     [exec]   File "/usr/lib64/python3.6/urllib/request.py", line 532, in open
     [exec]     response = meth(req, response)
     [exec]   File "/usr/lib64/python3.6/urllib/request.py", line 642, in http_response
     [exec]     'http', request, response, code, msg, hdrs)
     [exec]   File "/usr/lib64/python3.6/urllib/request.py", line 570, in error
     [exec]     return self._call_chain(*args)
     [exec]   File "/usr/lib64/python3.6/urllib/request.py", line 504, in _call_chain
     [exec]     result = func(*args)
     [exec]   File "/usr/lib64/python3.6/urllib/request.py", line 650, in http_error_default
     [exec]     raise HTTPError(req.full_url, code, msg, hdrs, fp)
     [exec] urllib.error.HTTPError: HTTP Error 502: Bad Gateway