Protect against undeclared variable Exception when bsddb is not present.
Protect against undeclared variable Exception when bsddb is not present.

RDFLib/rdflib,yingerj/rdflib,dbs/rdflib,armandobs14/rdflib,ssssam/rdflib,marma/rdflib,avorio/rdflib,RDFLib/rdflib,avorio/rdflib,ssssam/rdflib,avorio/rdflib,dbs/rdflib,yingerj/rdflib,armandobs14/rdflib,armandobs14/rdflib,marma/rdflib,ssssam/rdflib,dbs/rdflib,yingerj/rdflib,yingerj/rdflib,armandobs14/rdflib,RDFLib/rdflib,marma/rdflib,marma/rdflib,dbs/rdflib,ssssam/rdflib,RDFLib/rdflib,avorio/rdflib