Rollup merge of #47387 - Rantanen:linkchecker-error-msg, r=steveklabnik

Report errors instead of panic!() when linkcheck encounters absolute paths

The RBE contained some absolute links that failed the link check in #46196. Diagnosing these issues was needlessly complicated, thanks to the linkchecker just panicing instead of reporting proper errors.

This PR replaces the panic with a proper `*errors = true` + error message handling.

The linkchecker itself doesn't have any tests so I intentionally didn't touch anything else than the code that previously did the `panic!()`. A small code quality improvement might be made by binding the `Path::new(base).join(url)` into a variable before the for-loop and using this resolved url in both the for loop and the error message.

r? @steveklabnik

(If not for any other reason than having r on the #46196.)