Experimental support for #21

[njjewers]

A quick implementation of #21. This is accomplished by extending GrammarSyntaxError to provide more relevant messages than the default "expected" messages that Ohm gives. This is, in my opinion, the most appropriate place to put it, as this exception is only used for failure to create user grammars, and this keeps all ohm-specific parse failure messages separate from the rest of Ohm's logic.

I won't lie, I'm not entirely satisfied with it quite yet, especially the changes I made to the visualizer to support taking this from the exception. Perhaps it might be better to integrate this elsewhere.