#21 improve feedback on only or nothing sentence

[Anonymous]

When trying to enter a sentence with a 'only or nothing' list like:

'Every Pond only is connected to a River, a Stream, a Canal or nothing.'
if you forget the 'only' keyword, the parser will not understand the whole sentence, but won't give any feedback about what is wrong with the sentence. It's very hard to notice for a user who is not well acquainted with the rabbit patterns.