Show a warning when relative URI used in `sub`

This implementation has a mechanism for allowing relative URIs in `sub` requests. While the spec leaves out of scope what should happen when a client sends such an incorrect request, the server should "complain about any deviations from the spec in a way to help and to motivate the producers to adhere to it better" (quote from https://www.w3.org/DesignIssues/HTML-XML#grouchy)