The same message was used for "up" and "down" permission failures, but
"set" works better for both. Suggesting "up --operator" for a "down"
permission failure was confusing.
It's not like the latter command works in one shot anyway.
Fixes#16008
Change-Id: I6e4225ef06ce2d8e19c40bece8104e254c2aa525
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>