A couple of small scripting/XFA-related tweaks in the worker-code

- Use `PDFManager.ensureDoc`, rather than `PDFManager.ensure`, in a couple of spots in the code. If there exists a short-hand format, we should obviously use it whenever possible.

 - Fix a unit-test helper, to account for the previous changes. (Also, converts a function to be `async` instead.)

 - Add one more exists-check in `PDFDocument.loadXfaFonts`, which I missed to suggest in PR 13146, to prevent any possible errors if the method is ever called in a situation where it shouldn't be.
   Also, print a warning if the actual font-loading fails since that could help future debugging. (Finally, reduce overall indentation in the loop.)

 - Slightly unrelated, but make a small tweak of a comment in `src/core/fonts.js` to reduce possible confusion.

src/core/worker.js

@@ -195,7 +195,7 @@ class WorkerMessageHandler {
       ]);
 
       if (isPureXfa) {
-        const task = new WorkerTask("Load fonts for Xfa");
+        const task = new WorkerTask("loadXfaFonts");
         startWorkerTask(task);
         await pdfManager
           .loadXfaFonts(handler, task)