[ORC] Fix fall-through in error case in EPCGenericDylibManager::lookupAsync.

In the event of a serialization error (e.g. due to a network dropout) we should
only run the Complete handler once, passing the serialization error value.

No test-case: This would require a deliberately injected failure in a
remote-JIT test and we don't have the infrastructure for that at the moment.

rdar://126772381