Don't swallow docker build errors
Don't swallow docker build errors

Apparently the log generator can throw exceptions which we currently
ignore. Example:

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/atomic_reactor-1.6.14-py2.7.egg/atomic_reactor/inner.py", line 339, in build_docker_image
    build_result = self.builder.build()
  File "/usr/lib/python2.7/site-packages/atomic_reactor-1.6.14-py2.7.egg/atomic_reactor/build.py", line 122, in build
    command_result = wait_for_command(logs_gen)  # wait for build to finish
  File "/usr/lib/python2.7/site-packages/atomic_reactor-1.6.14-py2.7.egg/atomic_reactor/util.py", line 210, in wait_for_command
    item = next(logs_generator)  # py2 & 3 compat
  File "/usr/lib/python2.7/site-packages/docker/client.py", line 230, in _stream_helper
    yield self._result(response)
  File "/usr/lib/python2.7/site-packages/docker/client.py", line 150, in _result
    self._raise_for_status(response)
  File "/usr/lib/python2.7/site-packages/docker/client.py", line 146, in _raise_for_status
    raise errors.APIError(e, response, explanation=explanation)
APIError: 500 Server Error: Internal Server Error ("ENV must have two arguments")

projectatomic/atomic-reactor,fr34k8/atomic-reactor,vrutkovs/atomic-reactor,fr34k8/atomic-reactor,jarodwilson/atomic-reactor,DBuildService/atomic-reactor,projectatomic/atomic-reactor,maxamillion/atomic-reactor,jarodwilson/atomic-reactor,DBuildService/atomic-reactor,maxamillion/atomic-reactor,vrutkovs/atomic-reactor