Log traceback for initialize connection failure
Log traceback for initialize connection failure

Currently the manager logs at 'error' level when driver
initialize_connection fails before throwing VolumeBackendAPIException.
The original traceback will not be logged and this would make debugging
difficult. This patch changes the log level to 'exception' to log the
traceback before raising the exception.

Change-Id: I0e981120dee165a341c9015f62738e3b375afb00
Closes-bug: #1599052

bswartz/cinder,openstack/cinder,ge0rgi/cinder,openstack/cinder,cloudbase/cinder,phenoxim/cinder,phenoxim/cinder,bswartz/cinder,Datera/cinder,Hybrid-Cloud/cinder,j-griffith/cinder,NetApp/cinder,Datera/cinder,eharney/cinder,Nexenta/cinder,NetApp/cinder,Nexenta/cinder,Hybrid-Cloud/cinder,eharney/cinder,mahak/cinder,j-griffith/cinder,cloudbase/cinder,mahak/cinder