LibJS: Change wording of ErrorType::NotA to be independent of context

Currently, we have NotA and NotAn, to be used dependent on whether the
following word begins with a vowel or not. To avoid this, change the
wording on NotA to be independent of this context.

Userland/Libraries/LibJS/Tests/builtins/Promise/Promise.prototype.then.js

@@ -6,7 +6,7 @@ describe("errors", () => {
     test("this value must be a Promise", () => {
         expect(() => {
             Promise.prototype.then.call({});
-        }).toThrowWithMessage(TypeError, "Not a Promise");
+        }).toThrowWithMessage(TypeError, "Not an object of type Promise");
     });
 });