rawmidi: stream_pair: report error about write-only file descriptor

rawmidi: stream_pair: report error about write-only file descriptor

In design of ALSA rawmidi interface, one character device can handle
operations for both capture and playback. Applications can indicate
usage of both or either by flags of open system call.

The libalsarawmidi library checks the flag by fcntl(2) system call to
create GSource for capture. When the file descriptor is not configured
for read, the call returns error.

This commit handles the error for local error reporting.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>