More verbose response error messages when director registration failed

[haoming29]

When an origin or cache fail to register at the director (which it does for every 15s) due to token issues, the only error message showing up at the origin/cache end is

XRootD server advertise failed: Error during director registration: Authorization token verification failed.

which doesn't help a lot for origin/cache admins to debug and report the issue. We should give more verbose message back in the response without leaking any credentials.