LibJS: Consolidate error messages into ErrorTypes.h

Now, exceptions can be thrown with
interpreter.throw_exception<T>(ErrorType:TYPE, "format", "args",
"here").

Libraries/LibJS/Tests/in-operator-basic.js

@@ -6,7 +6,7 @@ try {
             "prop" in value;
         }, {
             error: TypeError,
-            message: "'in' operator must be used on object"
+            message: "'in' operator must be used on an object"
         });
     });