Don't rewrite the NotFound error message

When user updates an image with a nonexisted location, it's
expected to run into location not found exception. However,
because the error message is rewritten in v2 image update,
so user will get below wrong message:
Failed to find image <image-id> to update

This fix will remove the rewritten message, given the NotFound
message has been set at: glance/db/sqlalchemy/api.py#L159

Closes-Bug: #1265448

Change-Id: I4e5fbbe40ce0fd13e828a40a424d4fcdc66a236b
Merge "Don't rewrite the NotFound error message"