Previously, running `add/remove/revoke-keys` without passing any keys
would fail with an unhelpful error:
```console
$ tailscale lock revoke-keys
generation of recovery AUM failed: sending generate-recovery-aum: 500 Internal Server Error: no provided key is currently trusted
```
or
```console
$ tailscale lock revoke-keys
generation of recovery AUM failed: sending generate-recovery-aum: 500 Internal Server Error: network-lock is not active
```
Now they fail with a more useful error:
```console
$ tailscale lock revoke-keys
missing argument, expected one or more tailnet lock keys
```
Fixes#19130
Change-Id: I9d81fe2f5b92a335854e71cbc6928e7e77e537e3
Signed-off-by: Alex Chan <alexc@tailscale.com>
edb2be1a01