It'd be useful if URLs were always included in the error that the ErrorSink receives.
For example the error returned here

httprc.Resource.Sync: failed to execute HTTP request: Get "https://invalid": dial tcp: lookup invalid: no such host

whereas the error returned here

httprc.Resource.Sync: unexpected status code (status code=404)

I'm using this library via lestrrat-go/jwx and in setups with many JWKS URLs in the cache, it'd be helpful to immediately spot the one that has problems from the error logs.