fail if RDF extension was not loaded
fail if RDF extension was not loaded

This indicates that either an RDF extension was never loaded or that it
failed to load. The error message delivered in this commit cannot clarify
this distinction.

closes #50

OpenBEL/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,abargnesi/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,abargnesi/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,abargnesi/bel.rb,nbargnesi/bel.rb,abargnesi/bel.rb