Rather than printing `unknown subcommand: drive` for any Taildrive
commands run in the macOS GUI, print an error message directing the user
to the GUI client and the docs page.
Updates #17210Fixes#18823
Change-Id: I6435007b5911baee79274b56e3ee101e6bb6d809
Signed-off-by: Alex Chan <alexc@tailscale.com>