fftools/ffmpeg: Log exit code on exit

When viewing logs, there are situations where it is not entirely
clear whether ffmpeg CLI has exited gracefully. The two primary cases
are

- A crash/segfault has occured
  Windows for example doesn't output any message to the calling shell
- The process has been terminated (e.g. killed externally)

Printing a message on exit provides a reliable indication that the
process has exited normally.
Printing the exit code is useful as it usually remains invisible
and unnoticed by users running FFmpeg from a shell.

Signed-off-by: softworkz <softworkz@hotmail.com>

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