URLFile: raise exception if remote URL doesn't exist when using cache (#24432)

* URLFile returns empty bytes if using cache and remote file doesn't exist

* better exception

* assert on cached files

tools/lib/url_file.py

@@ -87,6 +87,7 @@ class URLFile:
 
     file_begin = self._pos
     file_end = self._pos + ll if ll is not None else self.get_length()
+    assert file_end != -1, f"Remote file is empty or doesn't exist: {self._url}"
     #  We have to align with chunks we store. Position is the begginiing of the latest chunk that starts before or at our file
     position = (file_begin // CHUNK_SIZE) * CHUNK_SIZE
     response = b""