sync: Fix misleading message when the value of `--exercise` lacks canonical data

### Example
Give `--exercise` an exercise that doesn't have canonical data:
```
configlet sync --exercise robot-name --tests
```

### Current behaviour
The program outputs:
```
Checking exercises...
The `robot-name` exercise has up-to-date tests!
```

### Expected behaviour
We could show something like:
```
Warning: exercise 'robot-name' does not have canonical test data.
```