Fixes attempted loading of unavailable gems

The begin/rescue/end block here seems to only exist to handle problems with the loading of RDF formats.

However, since `LoadError` is not a subclass of `StandardError`, the existing code wouldn't have caught cases where the JSON-LD gem wasn't available. And because `json-ld` is only a development dependency of `rdf-rdfa` and not a runtime one, this could bite unsuspecting users trying to parse a document with embedded JSON linked data - in my case it was http://www.businessweek.com/articles/2014-11-21/boeings-777-problem-delta-and-other-airlines-want-newer-planes.

    > LoadError.ancestors
     => [LoadError, ScriptError, Exception, Object, Kernel]

I'm not sure how to write a test around gems not being available in the load path, otherwise I would have.