cmd/tailscale: make ping --until-direct require direct connection to exit 0

If --until-direct is set, the goal is to make a direct connection.
If we failed at that, say so, and exit with an error.

RELNOTE=tailscale ping --until-direct (the default) now exits with
a non-zero exit code if no direct connection was established.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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