Provide more info around possible CRAN closure

[jennybc]

Closes #2700

[jennybc]

I'm doing this during the CRAN summer vacation and here's what submit_cran() shows at this very moment, with this PR:

Error in `submit_cran()`:
! Can't submit to CRAN right now.
ℹ Request to <https://xmpalantir.wu.ac.at/cransubmit/index2.php> failed.
ℹ Check <https://cran.r-project.org> to see if there is a planned closure or known outage.
Caused by error in `httr2::req_perform()`:
! Failed to perform HTTP request.
Caused by error in `curl::curl_fetch_memory()`:
! Timeout was reached [xmpalantir.wu.ac.at]:
Connection timed out after 10001 milliseconds

Between my experience and that posted in #2700, the experience can vary from the above, to a 503 (I've seen this personally today), to a 404 (OP reported that). Clearly the submission queue can be "down" in various technically distinct ways.