Add ResourceNotFound exception

This is used by both the get and delete changes, and currently get
depends on delete for it. It should just be by itself.

Change-Id: Ic11f8cc50c0f49b2ae37a5e09624a549602b66c8

openstack/exceptions.py

@@ -90,6 +90,11 @@ class DuplicateResource(SDKException):
     pass
 
 
+class ResourceNotFound(NotFoundException):
+    """No resource exists with that name or id."""
+    pass
+
+
 class ResourceTimeout(SDKException):
     """Timeout waiting for resource."""
     pass