DOI translation shouldn't fail if some DOIs fail to resolve

[dstillman]

https://forums.zotero.org/discussion/117003/why-did-zotero-7-fail-to-retrieve-data

I'm not positive this is the connector and not the translator, but Zotero is picking up an invalid DOI from an embedded link (because it can't tell where the actual DOI ends), and then it's getting a 404 for the content-negotiation request, and that seems to be failing the translator. An invalid DOI should just be skipped, and as long as there are other DOIs, the select-items window should still appear with the others.