docker: Handle exceptions when looking for docker command

docker: Handle exceptions when looking for docker command

When trying to run docker tests on a host without the docker
command,  we get the following Python backtrace:

  $ make docker-test-quick@centos6 V=1
  .../qemu/tests/docker/docker.py build qemu:centos6 .../qemu/tests/docker/dockerfiles/centos6.docker
  Traceback (most recent call last):
    File ".../qemu/tests/docker/docker.py", line 339, in <module>
      sys.exit(main())
    File ".../qemu/tests/docker/docker.py", line 336, in main
      return args.cmdobj.run(args, argv)
    File ".../qemu/tests/docker/docker.py", line 231, in run
      dkr = Docker()
    File ".../qemu/tests/docker/docker.py", line 98, in __init__
      self._command = _guess_docker_command()
    File ".../qemu/tests/docker/docker.py", line 41, in _guess_docker_command
      stdout=DEVNULL, stderr=DEVNULL) == 0:
    File "/usr/lib64/python2.7/subprocess.py", line 523, in call
      return Popen(*popenargs, **kwargs).wait()
    File "/usr/lib64/python2.7/subprocess.py", line 711, in __init__
      errread, errwrite)
    File "/usr/lib64/python2.7/subprocess.py", line 1343, in _execute_child
      raise child_exception
  OSError: [Errno 2] No such file or directory
  .../qemu/tests/docker/Makefile.include:47: recipe for target 'docker-image-centos6' failed
  make: *** [docker-image-centos6] Error 1

Change _guess_docker_command() to handle OSError exceptions
raised by subprocess.call(), so we will keep looking for other
commands and print a better error message.

New output will be:

  $ make docker-test-quick@centos6 V=1
  .../qemu/tests/docker/docker.py build qemu:centos6 .../qemu/tests/docker/dockerfiles/centos6.docker
  Traceback (most recent call last):
    File ".../qemu/tests/docker/docker.py", line 343, in <module>
      sys.exit(main())
    File ".../qemu/tests/docker/docker.py", line 340, in main
      return args.cmdobj.run(args, argv)
    File ".../qemu/tests/docker/docker.py", line 235, in run
      dkr = Docker()
    File ".../qemu/tests/docker/docker.py", line 102, in __init__
      self._command = _guess_docker_command()
    File ".../qemu/tests/docker/docker.py", line 49, in _guess_docker_command
      commands_txt)
  Exception: Cannot find working docker command. Tried:
    docker
    sudo -n docker
  .../qemu/tests/docker/Makefile.include:47: recipe for target 'docker-image-centos6' failed
  make: *** [docker-image-centos6] Error 1

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1474369559-16903-1-git-send-email-ehabkost@redhat.com>
[exceptions.OSError -> OSError and drop the import. - Fam]
Signed-off-by: Fam Zheng <famz@redhat.com>