Adds logic to `checkExitNodePrefsLocked` to return an error when
attempting to use exit nodes on a platform where this is not supported.
This mirrors logic that was added to error out when trying to use `ssh`
on an unsupported platform, and has very similar semantics.
Fixes https://github.com/tailscale/tailscale/issues/13724
Signed-off-by: Mario Minardi <mario@tailscale.com>
