Fix except in lvm.py

Fix except in lvm.py

Consolidation of brick exceptions into a single
brick exception.py file neglected to updated files
that were using it:
    (see change I577d403f02c2fd0d727d2694776008e1e9791453)

The result is the lvm driver was doing an except on lvm.VolumeGroupNotFound
which didn't exist anymore as it's in brick/exception now.

We still need to clean up exceptions all through here, and this still
isn't handled by service init anyway, but at least with this we are
raising a valid exception.

Change-Id: I588e9ac6548823b291d284f68f6133879bf3dfbf
Fixes: bug 1216122