Ensure that serializing of StructTree-data cannot fail during loading

I discovered that doing skip-cache re-reloading of https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf would *intermittently* cause (some of) the AnnotationLayers to break with errors printed in the console (see below).

In hindsight this bug is really obvious, however it took me quite some time to find it, since the `StructTreePage.prototype.serializable` getter will lookup various data and all of those cases can fail during loading when streaming and/or range requests are being used.

Finally, to prevent any future errors, ensure that the viewer won't break in these sort of situations.

```
Uncaught (in promise)
Object { message: "Missing data [19098296, 19098297)", name: "UnknownErrorException", details: "MissingDataException: Missing data [19098296, 19098297)", stack: "BaseExceptionClosure@resource://pdf.js/build/pdf.mjs:453:29\n@resource://pdf.js/build/pdf.mjs:456:2\n" }
viewer.mjs:8801:55

\#renderAnnotationLayer: "UnknownErrorException: Missing data [17552729, 17552730)". viewer.mjs:8737:15

Uncaught (in promise)
Object { message: "Missing data [17552729, 17552730)", name: "UnknownErrorException", details: "MissingDataException: Missing data [17552729, 17552730)", stack: "BaseExceptionClosure@resource://pdf.js/build/pdf.mjs:453:29\n@resource://pdf.js/build/pdf.mjs:456:2\n" }
viewer.mjs:8801:55
```

src/core/document.js

@@ -707,7 +707,7 @@ class Page {
     const structTree = await this.pdfManager.ensure(this, "_parseStructTree", [
       structTreeRoot,
     ]);
-    return structTree.serializable;
+    return this.pdfManager.ensure(structTree, "serializable");
   }
 
   /**