It would be nice to assume this but CompileFetchedModule can potentially fail for reasons that
are not JS-related.
Also check for the exception being |undefined| which would cause
ModuleScript::HasParseError to return false. Such an exception should never be
thrown by parsing but check for it just in case.
Add a diagnostic assert to check module script state is as we expect to
hopefully catch related problems sooner.
Differential Revision: https://phabricator.services.mozilla.com/D160788
5cd6b9919e