fix Scope::push block type error message

lib/6to5/traverse/scope.js

@@ -112,7 +112,7 @@ Scope.prototype.push = function (name, id, init) {
       init: init
     };
   } else {
-    throw new Error("wtf");
+    throw new TypeError("cannot add a declaration here in node type " + block.type);
   }
 };