cmd/tailscale: print a helpful error for Taildrive CLI on macOS GUI

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 #17210
Fixes #18823

Change-Id: I6435007b5911baee79274b56e3ee101e6bb6d809
Signed-off-by: Alex Chan <alexc@tailscale.com>
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