Throw a better exception when a constant bnode is used

[bcogrel]

The property rr:constant (https://www.w3.org/TR/r2rml/#constant) does not accept blank nodes, only IRIs or literals according to the context.

Invalid triples maps like

<urn:MAPID-institution> a rr:TriplesMap;
  rr:logicalTable [ a rr:R2RMLView;
      rr:sqlQuery "SELECT prof_id FROM professors;"
    ];
  rr:predicateObjectMap [ a rr:PredicateObjectMap;
      rr:objectMap [ a rr:ObjectMap, rr:TermMap;
          rr:constant _:mit
        ];
      rr:predicate :institution
    ];
  rr:subjectMap [ a rr:SubjectMap, rr:TermMap;
      rr:template "professor_{prof_id}";
      rr:termType rr:BlankNode
    ] .

<urn:MAPID-mit> a rr:TriplesMap;
  rr:logicalTable [ a rr:R2RMLView;
      rr:sqlQuery "SELECT prof_id FROM professors;"
    ];
  rr:subjectMap [ a rr:SubjectMap, rr:TermMap;
      rr:class :Institution;
      rr:constant _:mit
    ] .

triggers casting exceptions instead of saying that the mapping is invalid