[threadpool-io] Throw if backend won't be able to register a FD (#16396)
The poll+select i/o selector backend can't handle file descriptor ids greater
than FD_SETSIZE. This can happen if too many files are open and we want to
wait on it.
Previously, mono would fail in the i/o selector thread by which point it was
too late to do anything.
With this change we will fail eagerly on the thread that calls IOSelector.Add
by throwing a NotSupportedException.
Addresses https://github.com/mono/mono/issues/15931