[backend] log real cause if ansible crash
    
    addressing:
    Process worker-builder:
    Traceback (most recent call last):
      File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
        self.run()
      File "/usr/share/copr/backend/dispatcher.py", line 302, in run
        ip = self.spawn_instance()
      File "/usr/share/copr/backend/dispatcher.py", line 165, in spawn_instance
        shell=True)
      File "/usr/lib64/python2.7/subprocess.py", line 575, in check_output
        raise CalledProcessError(retcode, cmd, output=output)
    CalledProcessError: Command 'ansible-playbook -c ssh /home/copr/provision/builderpb.yml' returned non-zero exit status 2