Broadening exception for fallback on API call failures.

We are now experiencing ServiceUnavailable errors which are not a
child of ClientError, and therefore not being handled.
GoogleAPIError is the first common ancestor of these two error
types, and is the most generic error that is still specific to
Google API calls.

TESTING=None
BUG=175343328

Change-Id: Idbf75728e590bbe56138b156c192eee36402bedd
1 file changed