Correctly generate errors in ClojureScript client code:
 - The previous approach tried to use cljx in a macro, which doesn't work
 - Schema.macros has a correct approach based on checking the environment of the macro
 - Use that approach; this incidentally changes exception types to RuntimeException,
   but doesn't seem like a big deal.
Merge pull request #45 from Prismatic/proper-cross-platform-errors

Correctly generate errors in ClojureScript client code