Fixed error message for recoverable exceptions

Issue was introduced by https://review.openstack.org/#/c/76354/
In case of recovarable exceptions message is None and exc_value
need to be used. But after encoding.force_unicode(message) message
becomes u'None' and all checks on "if message" pass. This result
in popup screen with 'None' text instead of problem description.

Change-Id: I7bc2dd02f97ba2bb10a7fadd2c1b86b011eb5967
Closes-Bug: #1350110
Merge "Fixed error message for recoverable exceptions"