Catch 404 from CrossRef instead of catching all errors

[nsunami]

As I mentioned in #747 and #785 (comment), the db mutation for adding a reference module (createReferenceModule) catches all errors trying to use CrossRef API, and then resolving to use DataCite.

This logic makes a bit harder to debug, since the console will be flooded with the error message from DataCite. I think it would be better to catch 404 from CrossRef, and then resolving to use DataCite. Any other errors will stop the execution, and be reported in the console accordingly.