Add exception SnapshotIsBusy to be handled as VolumeIsBusy.

This is helpful when driver can not delete snapshot if it has volumes
created from it.
Add tests for both VolumeIsBusy and SnapshotIsBusy cases.

Change-Id: I3fcc6b1ffdfbc7e22ed30ac832db500eb3260d04

nova/exception.py

@@ -447,6 +447,11 @@ class VolumeIsBusy(NovaException):
     message = _("deleting volume %(volume_name)s that has snapshot")
 
 
+class SnapshotIsBusy(NovaException):
+    message = _("deleting snapshot %(snapshot_name)s that has "
+                "dependent volumes")
+
+
 class ISCSITargetNotFoundForVolume(NotFound):
     message = _("No target id found for volume %(volume_id)s.")