Raise exception if volume snapshot id not found instead of return

In V2 API, if show or delete a snapshot with an unknown id, exception
will be returned instead of raised. Even though correct error
will be returned to client, it's not aligned to other API layer
code, this patch modify it by raise exception instead of return.

Change-Id: I22d4565ca08e1c0ed5041ce903bf6ba918c14cfd
Closes-Bug: #1279189
Merge "Raise exception if volume snapshot id not found instead of return"