Better error message for three non-distinct variables

[hannahbast]

Graph patterns like SELECT * WHERE { ?s ?p ?s } are currently not supported by QLever. The error message so far looked like a bug ("Assertion !lastRow.empty() failed"). Now there is a proper error message with a recommendation to use a FILTER, see below.

Addresses https://github.com/ad-freiburg/qlever/issues/1283.

An equivalent formulation for the query above is SELECT * WHERE { ?s ?p ?o FILTER (?s = ?o) }. However, that only works if all triples fit into RAM. This is a separate issue (it requires lazy processing of FILTER operations), which is on our list.