Return nil, err on invalid mask

This:
```
ipmatcher.new({"0.0.0.0/asdfsdf", "0.0.0.0/-1", "0.0.0.0/343546w3462343" })
```

return a table, while this returns `nil, "invalid ip address: blaa0.0.0.0"`:
```
ipmatcher.new({"blaa0.0.0.0/asdfsdf", "0.0.0.0/-1", "0.0.0.0/343546w3462343" })
```

Perhaps the first should also return `nil, "invalid subnet mask:  asdfsdf"`.

What do you think? Thanks for the nice lib!