Fix raise on Snapshot initialization

We were calling "raise" directly when initialization of a Snapshot
failed due to the backend provided, but we were not within an "Except"
caluse, so it was incorrect.

This patch ensure we raise an appropriate ValueError exception.

HISTORY.rst

@@ -2,6 +2,13 @@
 History
 =======
 
+0.3.9 (2019-02-18)
+------------------
+
+- Bug fixes:
+
+  - Incorrect raise on Snapshot initialization backend errors.
+
 0.3.8 (2019-02-15)
 ------------------
 

cinderlib/objects.py

@@ -860,10 +860,10 @@ class Snapshot(NamedObject, LazyVolumeAttr):
                 backend = param_backend and param_backend.id
 
         if not (backend or param_backend):
-            raise
+            raise ValueError('Backend not provided')
 
         if backend and param_backend and param_backend.id != backend:
-            raise
+            raise ValueError("Multiple backends provided and they don't match")
 
         super(Snapshot, self).__init__(backend=param_backend or backend,
                                        **kwargs)