Fix incorrect use of ServerError

ServerError is being incorrectly used to report an internal error
leading to an error formatting the response, as there's no body
specified to the format string.  Use exception.Error instead and
adjust the message to make the user-visible response clearer.

Fixes bug #1214457

Change-Id: I83435686170c4f40658968a35f1faa303f50f4a4
Merge "Fix incorrect use of ServerError"