Remove custom error message for image 404

For reasons covered in the discussion on #38, I feel a custom error message is unnecessary. The default exception will contain the message from the server, which is explicit enough:

    >>> d.create_container('ubuntu', 'bash')
    {u'Id': u'7cad7baac3dc'}

    >>> d.create_container('doesntexist', 'bash')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "docker/client.py", line 237, in create_container
        return self.create_container_from_config(config)
      File "docker/client.py", line 242, in create_container_from_config
        return self._result(res, True)
      File "docker/client.py", line 82, in _result
        self._raise_for_status(response)
      File "docker/client.py", line 79, in _raise_for_status
        raise APIError(e, response=response, explanation=explanation)
    docker.client.APIError: 404 Client Error: Not Found ("No such image: doesntexist (tag: latest)")