Test to ensure that creator is valid

Providing a better error message is always useful

src/core/store.coffee

@@ -102,6 +102,9 @@ module.exports = class kb.Store
     return obj if creator.models_only
     return observable if observable = @find(obj, creator)
 
+    unless _.isFunction(creator.create or creator)
+      throw new Error "Invalid factory for \"#{options.path}\""
+
     observable = kb.ignore =>
       options = _.defaults({store: @, creator: creator}, options) # set our own creator so we can register ourselves above
       observable = if creator.create then creator.create(obj, options) else new creator(obj, options)