Handle minzoom > maxzoom better in generation

Mixing min/max zoom gives the error
```
  File "/opt/tilekiln/lib/python3.11/site-packages/tilekiln/scripts/generate.py", line 91, in zooms
    threads = min(num_threads, len(tiles))  # No point in more threads than tiles
                               ^^^^^^^^^^
ValueError: __len__() should return >= 0
```

This should be checked earlier and a better error given