Use loader signals for progress and fix error handler arguments.

PIXI.Loader#on is a no-op since pixi.js 5.3, so the progress callback
in PIXIApp.loadTextures was never called. Also PIXI.Loader#onError
passes (error, loader, resource), so log the failed url and error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/pixi/deepzoom/loader.js

@@ -143,8 +143,8 @@ export class PIXITileLoader extends TileLoader {
         }
     }
 
-    _onError(loader, error) {
-        console.warn('Cannot load', error)
+    _onError(error, loader, resource) {
+        console.warn('Cannot load', resource ? resource.url : '', error)
     }
 
     /** Private method. Called by the PIXI loader after each successfull