Handle terminate_connection() exception in volume manager

Due to the fact that we sometimes need to manually terminate a volume's
connection through volume api, it's possile that these backend drivers
throw exceptions while doing that. Currently exceptions are bubbled up to
volume API not being handled. This patch logs exception in volume manager
and then raises VolumeBackendAPIException to caller.

Change-Id: If809f97998f52516af09ec21b3052b67d3a62f36
Closes-bug: #1263820
Merge "Handle terminate_connection() exception in volume manager"