add link to docs in fetch error messages (in case of CORS)

CORS issues are tricky and confusing. it would be great if we could let people know if they are hitting a CORS error in particular (rather than a fetch error in general) when they run into a CORS problem, and direct them to help / workarounds / more info.

(in investigating this, we might find that there are other kinds of errors we can surface/specify)

cc @hamilton