<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def condor_stop(external_id):
 """ Stop running condor job and return a failure_message if this fails. """

    failure_message = None
    try:
        check_call(('condor_rm', external_id))
    except CalledProcessError:
        failure_message = "condor_rm failed"
    except Exception as e:
        "error encountered calling condor_rm: %s" % e
    return failure_message