Referencing a non-existent reducer should cause an error

[emlyn]

If you mis-spell the name of a reducer function, it will be ignored silently, e.g.:

(let [fsm (a/compile [:foo :bar (a/$ :baar)]
                     {:reducers {:bar (fn [state _] (assoc state :bar true))}})]
  (reduce (partial a/advance fsm)
          {}
          [:foo :bar]))

I believe it's because of this line in the eval version, and this one in the base version, throwing away any nils after looking up the reducer in the reducers map. Wouldn't it be better to throw some sort of error if a requested reducer does not exist?