flatpak-proxy: Tell where "Invalid message header format" comes from

"Invalid message header format" was printed without more details.
Printing where the invalid message header came from might help in
debugging.

flatpak-proxy.c

@@ -2170,7 +2170,7 @@ got_buffer_from_client (FlatpakProxyClient *client, ProxySide *side, Buffer *buf
       header = parse_header (buffer, client->serial_offset, 0, 0);
       if (header == NULL)
         {
-          g_warning ("Invalid message header format");
+          g_warning ("Invalid message header format from client");
           side_closed (side);
           buffer_unref (buffer);
           return;
@@ -2340,7 +2340,7 @@ got_buffer_from_bus (FlatpakProxyClient *client, ProxySide *side, Buffer *buffer
       header = parse_header (buffer, 0, client->serial_offset, client->hello_serial);
       if (header == NULL)
         {
-          g_warning ("Invalid message header format");
+          g_warning ("Invalid message header format from bus");
           buffer_unref (buffer);
           side_closed (side);
           return;