(defn form-spec [spec err-format]
  (with-meta (s/spec spec) {:err-format err-format}))

(s/def ::number (form-spec int? (fn [k v] (str (name k) " is not a valid number: " v))))
(s/def :address/apt (form-spec ::number (fn [_ v] (str "wrong apartment number: " v))))
(s/def :address/strno ::number)
(s/def :address/street string?)

(s/def ::addr (s/keys :req-un [:address/apt :address/street :address/strno]))

(defn err [k v] (str (name k) " is invalid: " v))

(->> (s/explain-data ::addr {:apt "1" :street 23 :strno "stt"})
     ::s/problems
     (map (fn [{:keys [path val via]}]
            (let [errfn (-> via last s/get-spec meta :err-format (or err))]
              (errfn (last path) val)))))
;; => ("wrong apartment number: 1" "street is invalid: 23" "strno is not a valid number: stt")