Handle nio.ThumbnailError cases

Return a 1x1 transparent PNG for now.

TODO.md

@@ -3,8 +3,6 @@
 - Accent color from background
 - Reduce messages ListView cacheBuffer height once http thumbnails
   downloading is implemented 
-- Button can get "hoverEnabled: false" to let HoverHandlers work
-- Handle thumbnail response status 400
 - Improve avatar tooltips position, add stuff to room tooltips (last msg?)
 - Accept drag and dropping a picture in account settings to set avatar
 - When all the events loaded on beginning in a room are name/avatar changes,
@@ -18,6 +16,7 @@
 - Refactoring
   - Don't bake in size properties for components
   - Unfinished work in button-refactor branch
+    - Button can get "hoverEnabled: false" to let HoverHandlers work
   - Room Sidepane
 
 - Bug fixes

src/python/image_provider.py

@@ -92,6 +92,13 @@ class Thumbnail:
             height      = self.server_size[1],
             method      = self.resize_method,
         )
+
+        if isinstance(response, nio.ThumbnailError):
+            # Return a transparent 1x1 PNG
+            with BytesIO() as img_out:
+                PILImage.new("RGBA", (1, 1), (0, 0, 0, 0)).save(img_out, "PNG")
+                return img_out.getvalue()
+
         body = response.body
 
         if response.content_type not in ("image/jpeg", "image/png"):