Better error message for an assertion

In some unexpected situations BAT would just show that the
`bfile_pp is not None` assertion failed. Now it also shows which file is
the one causing this issue, aiding in debugging the situation.

blender_asset_tracer/pack/__init__.py

@@ -475,7 +475,8 @@ class Packer:
             # It is *not* used for any disk I/O, since the file may not even
             # exist on the local filesystem.
             bfile_pp = action.new_path
-            assert bfile_pp is not None
+            assert bfile_pp is not None, \
+                f"Action {action.path_action.name} on {bfile_path} has no final path set, unable to process"
 
             # Use tempfile to create a unique name in our temporary directoy.
             # The file should be deleted when self.close() is called, and not