Clearer error message for unparseable LR items

[michael-schwarz]

I was checking some features and made a typo and had a . instead of a , for separating the lookahead.
The error message from the tool was not too helpful in debugging this issue.

It would be nice if there was some sort of clearer error message if an item is malformed (i.e. it can not even be parsed as an LR(0/1) item)

Input file: Saved Automaton, rename to *.xml graph.txt

[michael-schwarz]

Btw, fixing this is of course completely voluntary and will not impact grading, as the official deadline is already over 😄