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.)
29c2aa7932