Add javascript error reporting

Maybe there should be an `else` here that defines `BugSnag.notifyException` as a no-op. That way, if `ENV['BUGSNAG_JAVASCRIPT_API_KEY']` evaluates to false, line 28 of `app/assets/javascripts/initializers/error_handler.js.coffee` won't blow up -- and we don't have an error in our error handling code :smile: