messages get on a nonexistent channel returns [] with exit 0

## Repro (stock CLI v0.5.2)

```
buzz messages get --channel 00000000-0000-4000-8000-000000000000 --limit 1
```

Prints `[]` and exits 0. That is indistinguishable from a real empty channel.

## Expected

Nonexistent / unauthorized channel IDs should fail closed (nonzero exit and an error), not look like an empty history.

## Impact

Scripts cannot tell "no messages yet" from "wrong UUID" or "not a member".