Add visible indicator when downloading files

Downloading file messages will now show a transfer control above the
composer, similar to uploads. Measuring the progress or pausing the
operation is not possible yet.

src/backend/errors.py

@@ -90,6 +90,11 @@ class MatrixTooLarge(MatrixError):
     m_code:    str = "M_TOO_LARGE"
 
 
+@dataclass
+class MatrixBadGateway(MatrixError):
+    http_code: int           = 502
+    m_code:    Optional[str] = None
+
 @dataclass
 class MatrixBadGateway(MatrixError):
     http_code: int           = 502