When running as non-root non-operator user, you get this error:
```
$ tailscale serve 8080
Access denied: watch IPN bus access denied, must set ipn.NotifyNoPrivateKeys when not running as admin/root or operator
Use 'sudo tailscale serve 8080' or 'tailscale up --operator=$USER' to not require root.
```
It should fail, but the error message is confusing.
With this fix:
```
$ tailscale serve 8080
sending serve config: Access denied: serve config denied
Use 'sudo tailscale serve 8080' or 'tailscale up --operator=$USER' to not require root.
```
Updates #cleanup
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
fbfc3b7e51