Throw InvalidSnapshot for failed snap delete

When delete is called for a snapshot not in the "available" or
"error" state, throw an InvalidSnapshot exception rather than
InvalidVolume.

Change-Id: I47f55b3357d75b5c0c9a704aaf693f4764fd112c
Remove InvalidPortRange exception

The InvalidPortRange exception seems to be left over from the
initial split from Nova.

Change-Id: Ife03ac70f6ba80273c1c21e8d2c509b480a6fe80
Merge "Remove InvalidPortRange exception"