Tolerate some errors as part of external updates

[oliverchang]

Currently, a returned error from the reproduction infrastructure causes the Testcase to be marked as invalid:

https://github.com/google/clusterfuzz/blob/ab7dbf2c01610b3f01d60f665d58cd1a6d611e4f/src/appengine/handlers/external_update.py#L87

We should introduce a retry here before we do so.