provide underlying error when addServer() fails

[aronatkins]

> rsconnect::addServer("https://this-is-not-a-valid-server")
Error in `rsconnect::addServer()`:
! `url` does not appear to be a Posit Connect server.
Run `rlang::last_trace()` to see where the error occurred.

We should provide the underlying error; it is available, but dropped.

[aronatkins]

#1198 shows one experiment that is often too noisy.