Halt the current step.

def remote_interruptCommand(self, stepId, why):
        """Halt the current step."""
        log.msg("asked to interrupt current command: {0}".format(why))
        self.activity()
        if not self.command:
            # TODO: just log it, a race could result in their interrupting a
            # command that wasn't actually running
            log.msg(" .. but none was running")
            return
        self.command.doInterrupt()