JS backend: report diagonstic message for unsupported element instead of throw exception.

#KT-6507 Fixed

compiler/testData/cli/js/diagnosticForUnhandledElements.kt

@@ -0,0 +1,18 @@
+package foo
+
+annotation class fancy
+
+[fancy]
+class Foo {
+    [fancy]
+    fun baz([fancy] foo : Int) : Int {
+        return [fancy] 1
+    }
+}
+
+fun test() {
+
+    class A {
+        fun foo(){}
+    }
+}