Give the reason of "expect fail" failure

[qiankehan]

The expect of command pass is common while expect fail is not. It is confused to debug a case failure when expect fail but success if no extra msg given:

2018-09-26 10:58:19,044 stacktrace       L0044 ERROR| Reproduced traceback from: /root/avocado/avocado/core/test.py:864
2018-09-26 10:58:19,044 stacktrace       L0047 ERROR| Traceback (most recent call last):
2018-09-26 10:58:19,044 stacktrace       L0047 ERROR|   File "/root/avocado-vt/avocado_vt/test.py", line 297, in runTest
2018-09-26 10:58:19,044 stacktrace       L0047 ERROR|     raise self.__status  # pylint: disable=E0702
2018-09-26 10:58:19,045 stacktrace       L0047 ERROR| TestFail: Run '/bin/virsh attach-disk --domain avocado-vt-vm1 --source /var/tmp/avocado_w4K5Mq/native.raw --target sda ${disk_attach_bus_option}' expect fail, but run successfully.

Could you please add extra msg why expect failure for all "expect fail" code?
@chunfuwen