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

Start by locating the fetch error handling and the existing documentation or help page for CORS workarounds. Confirm how CORS failures can be distinguished from other fetch errors, then update the error message with the relevant link and verify that the resulting message remains useful for general fetch failures.

Written by the indexing model from the issue text.