lava-dispatcher exits when test failed

[Yongqin Liu]

when the 0xbench test failed to run, lava-dispatcher will exit with run the test after 0xbench.
because lava-dispatcher failed to parse the error.
the job http://validation.linaro.org/lava-server/scheduler/job/11026/log_file.

the exception information:

Traceback (most recent call last):
  File "/srv/lava/instances/production/bin/lava-dispatch", line 77, in <module>
    job.run()
  File "/srv/lava/instances/production/local/lib/python2.7/site-packages/lava_dispatcher/job.py", line 141, in run
    (cmd['command'], unicode(err, 'ascii', 'replace'))
TypeError: coercing to Unicode: need string or buffer, OperationFailed found

with the following test script, this error can reproduce.
(pip)18:52:56 liuyq:lava-dispatcher$ cat /tmp/test.py
#!/usr/bin/python

from lava_dispatcher.client.base import OperationFailed

try:
    raise OperationFailed("Failed to run test case(%s) on device(%s) with return "
    "value: %s" % (test_name, session.dev_name, rc))
except Exception as err:
    pass
finally:
    err_msg = unicode(err, 'ascii', 'replace')
    print 'err_msg=%s' % err_msg
(pip)18:53:00 liuyq:lava-dispatcher$ python /tmp/test.py
Traceback (most recent call last):
  File "/tmp/test.py", line 11, in <module>
    err_msg = unicode(err, 'ascii', 'replace')
TypeError: coercing to Unicode: need string or buffer, exceptions.NameError found
(pip)18:53:16 liuyq:lava-dispatcher$