Sparql parse error and  SPARQL queries from files

This pull request includes two changes and I'm happy to discuss whether they fit the ldif architecture.

The first one (f827709) is a bug fix. When you passed an invalid SPARQL query the import job would go into an infinite loop and there's no debug output which hints to that. This commit catches the SPARQL parser error and adds corresponding logging statements on debug level.

The second one (3663049) is a new feature. It allows to create more complex SPARQL queries for the SPARQL Import Job. Essentially it adds the specification of a SPARQL file in the SPARQL import job configuration. This sparql file is then used as the source of the query. It still has to have the three variables ?s ?p ?o and should not add any limits or offsets to the query.