Exception when downloading job log fails

[ruleant]

When network is unavailable when downloading a job log, an HTTPError exception is thrown.
Resolution : catch exception and schedule task to be retried later. (see #94)

Stacktrace :

 Jul 27 11:36:25 buildtimetrend app/worker.1: [2015-07-27 18:36:25,135: ERROR/MainProcess] Task tasks.process_travis_buildlog[feecba56-e283-425f-ba35-538677a6113f] raised unexpected: IOError() 
Jul 27 11:36:25 buildtimetrend app/worker.1: Traceback (most recent call last): 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/site-packages/celery/app/trace.py", line 240, in trace_task 
Jul 27 11:36:25 buildtimetrend app/worker.1:     R = retval = fun(*args, **kwargs) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/site-packages/celery/app/trace.py", line 438, in __protected_call__ 
Jul 27 11:36:25 buildtimetrend app/worker.1:     return self.run(*args, **kwargs) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/tasks.py", line 75, in process_travis_buildlog 
Jul 27 11:36:25 buildtimetrend app/worker.1:     for build_job in travis_data.process_build_jobs(): 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/site-packages/buildtimetrend/travis.py", line 358, in process_build_jobs 
Jul 27 11:36:25 buildtimetrend app/worker.1:     yield self.process_build_job(job_id) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/site-packages/buildtimetrend/travis.py", line 375, in process_build_job 
Jul 27 11:36:25 buildtimetrend app/worker.1:     self.parse_job_log(job_id) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/site-packages/buildtimetrend/travis.py", line 518, in parse_job_log 
Jul 27 11:36:25 buildtimetrend app/worker.1:     self.parse_job_log_stream(self.connector.download_job_log(job_id)) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/site-packages/buildtimetrend/travis.py", line 221, in download_job_log 
Jul 27 11:36:25 buildtimetrend app/worker.1:     return self._handle_request(request) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/site-packages/buildtimetrend/travis.py", line 260, in _handle_request 
Jul 27 11:36:25 buildtimetrend app/worker.1:     return opener.open(req) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/urllib2.py", line 437, in open 
Jul 27 11:36:25 buildtimetrend app/worker.1:     response = meth(req, response) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/urllib2.py", line 550, in http_response 
Jul 27 11:36:25 buildtimetrend app/worker.1:     'http', request, response, code, msg, hdrs) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/urllib2.py", line 475, in error 
Jul 27 11:36:25 buildtimetrend app/worker.1:     return self._call_chain(*args) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/urllib2.py", line 409, in _call_chain 
Jul 27 11:36:25 buildtimetrend app/worker.1:     result = func(*args) 
Jul 27 11:36:25 buildtimetrend app/worker.1:   File "/app/.heroku/python/lib/python2.7/urllib2.py", line 558, in http_error_default 
Jul 27 11:36:25 buildtimetrend app/worker.1:     raise HTTPError(req.get_full_url(), code, msg, hdrs, fp) 
Jul 27 11:36:25 buildtimetrend app/worker.1: HTTPError