JS: make kotlin.Throwable = Error. See KT-15312

compiler/testData/codegen/box/specialBuiltins/throwableCause.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    var t = Throwable("O", Throwable("K"))
+    if (t.message != "O" || t.cause?.message != "K") return "fail1"
+
+    t = Throwable(Throwable("OK"))
+    if (t.message != null || t.cause?.message != "OK") return "fail2"
+
+    return "OK"
+}