ffmpeg: check return value of avio_closep for progress report

avio_closep is not guaranteed to succeed, and its return value can
contain information regarding failure of preceding writes and silent
loss of data (man 2 close, man fclose). Users should know when the
progress was not successfully logged, and so a diagnostic is printed
here.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
