Interceptor return types are incorrect.

The return types should match the fetch spec, in particular the `requestError` and `responseErrors` should have valid return types (they do not appear to from what I can tell)

Start by locating the interceptor return-type declarations, especially requestError and responseErrors, and compare them with the Fetch specification. Done means both interceptor paths expose valid return types that match the specification; the issue does not name a file or test to run.

Written by the indexing model from the issue text.