Typos in queries need much better feedback / error msgs

[Buchhold]

Feel free to add to this list:

• PREFIX < VS REFIX < leads to empty results without error.
• missing quotes for <in-text> objects can cause QLever to ignore parts of the object
• Selected variables that do not occur in the query are ignored without error message

[niklas88]

I think this too is best solved by rewriting the SPARQL parser using proper token matching and recursive descent parsing as @joka921 is currently testing for Turtle input. This would also include converting literals to non-string types before they leave the parser.

[hannahbast]

Since the SPARQL queries are parsed via ANTLR, there are much better error messages and the QLever UI even highlights the offending part of the query.