Timeout when parser throws exception

[LaurensRietveld]

When the parser throws an exception (e.g. in this situation https://github.com/rdfhdt/hdt-cpp/issues/11) then no response status is returned. Would be nice to catch errors like this and return a 500 error message

[RubenVerborgh]

That's a bug indeed, incorrect HDT files exist, so we should be able to cope with them.