cmd/tailscale: improve `tailscale lock` error message if no keys

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>

0 files changed, 0 insertions, 0 deletions