fix show function FQN in instrumentation exception

src/malli/core.cljc

@@ -2528,7 +2528,12 @@
 (defn -register-function-schema!
   ([ns name ?schema data] (-register-function-schema! ns name ?schema data :clj function-schema))
   ([ns name ?schema data key f]
-   (swap! -function-schemas* assoc-in [key ns name] (merge data {:schema (f ?schema), :ns ns, :name name}))))
+   (try
+     (swap! -function-schemas* assoc-in [key ns name] (merge data {:schema (f ?schema), :ns ns, :name name}))
+     (catch #?(:clj Throwable :cljs :default) ex
+       (throw (ex-info
+                (str "Schema error when insrumenting function: " ns "/" name " - " (ex-message ex))
+                (ex-data ex)))))))
 
 #?(:clj
    (defmacro => [given-sym value]

test/malli/dev_err_test.clj

@@ -0,0 +1,27 @@
+(ns malli.dev-err-test
+  (:require [clojure.test :refer [deftest is testing]]
+            [malli.dev :as md]
+            [malli.dev.pretty :as pretty]))
+
+(defn plus-err
+  [x] (inc x))
+
+(defn ->plus-err [] plus-err)
+
+(deftest start!-err-test
+  (testing "malli.dev/start!"
+    (testing "without starting"
+      (is (thrown? ClassCastException ((->plus-err) "2")))
+      (is (= 7 ((->plus-err) 6))))
+
+    (testing "instrumentation shema error when starting"
+      ;; append metadata only during test to prevent conflicts with other tests
+      (alter-meta! #'plus-err #(assoc % :malli/schema [:=> [:cat [:vector]] [:int {:max 6}]]))
+      (try
+        (is (thrown-with-msg?
+             Exception #"Schema error when insrumenting function: malli.dev-err-test/plus-err - :malli.core/child-error"
+             (md/start! {:ns *ns*})))
+        (catch Throwable _
+          (md/stop!)))
+      (alter-meta! #'plus-err #(dissoc % :malli/schema)))))
+