_drain_stderr and _wait_until_ready both read from the same stderr pipe.
Starting the drain task before _wait_until_ready creates a race where the
drain task consumes aria2's early-exit error message before the startup
waiter can read it, resulting in an empty error message in the logs.
Also confirmed that --fsync does not exist as an aria2 option (exit code
28 = Invalid argument).
Will Miao
75fffc1e25