Fix loading m.image events in encrypted rooms

Fix bug introduced in 11fb32:

When loading an encrypted thumbnail, QML lacks the decryption dict for
half a second at first.

When calling python like this, python calls the wrong matrix API
for fetching the encrypted thumbnail, and the added retry code
would be forever stuck.

The retry code has been moved to QML, and now works for all HImage.

src/backend/media_cache.py

@@ -12,7 +12,6 @@ from dataclasses import dataclass, field
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, DefaultDict, Dict, Optional
 from urllib.parse import urlparse
-from .errors import MatrixError
 
 from PIL import Image as PILImage
 
@@ -134,20 +133,8 @@ class Media:
     async def create(self) -> Path:
         """Download and cache the media file to disk."""
 
-        retries = 0
-
-        while True:
-            try:
-                async with CONCURRENT_DOWNLOADS_LIMIT:
-                    data = await self._get_remote_data()
-            except MatrixError as err:
-                if err.http_code != 404 and err.http_code < 500:
-                    raise
-            else:
-                break
-
-            await asyncio.sleep(min(30, 0.2 * (2 ** (min(1000, retries) - 1))))
-            retries += 1
+        async with CONCURRENT_DOWNLOADS_LIMIT:
+            data = await self._get_remote_data()
 
         self.local_path.parent.mkdir(parents=True, exist_ok=True)