RBD: remove redundant exception log to reduce noise
RBD: remove redundant exception log to reduce noise

If RBDVolumeProxy fails to open the underlying image it reraises the
exception, so there is no need to log it unconditionally.  The caller
can gracefully handle this case without alarming the admin.

Change-Id: If5a9d014dbdd63a6e4659d700671c6ac6496b96a
(cherry picked from commit 71584d6852a6dec8d8d04c8f3cbddf0f114777af)

Datera/cinder,Datera/cinder