Handle initialize_connection() exception in volume manager

Due to the fact that several drivers require backend communication to fetch
connection information for a volume, it's possile that these driver throw
exceptions while doing that.  Currently exceptions can bubble up to volume
API not being handled.  This patch logs exception in volume manager and
then raises VolumeBackendAPIException to caller.

Change-Id: Ib3cc152e04ba029dd835a64b0cfb0a77b8a6828e
Closes-bug: 1256804
Merge "Handle initialize_connection() exception in volume manager"