[RecognizePDF] Don't skip ISBN search if a DOI search fails

Re https://forums.zotero.org/discussion/39713/error-in-metadata-extraction/

Currently, we assume that PDFs that have a DOI will not have an ISBN, but we've now seen a report where this hinders metadata extraction. The linked reports on forums have DOIs (issued by EU, which we currently cannot get metadata for) and ISBNs. We would be able to retrieve metadata using ISBNs (WorldCat has metadata for them), but we ignore them because we had found a DOI.

The translate change is just for convenience though, but IMO it makes sense.
I think this needs to be `var` to avoid breaking the bookmarklet in some browsers.