fix bad usage of exc_info=True

To use parameter exc_info in logging, here
(https://review.openstack.org/#/c/38077/) also uses some bad usage of
that. This patch will fix:
* use trackback.stacktrace() instead of exc_info
* use "Error: %s" % exc_info[1] (removing .message to follow PEP352)

Change-Id: Ia661ea9ce760f51359193904b77f7323434ed463
Merge "fix bad usage of exc_info=True"