Improve exception handling in 'ffmpeg_parse_info'

[mondeja]

These minor changes try to fix the #1487 possible issue and, if not fixes it, improves the error handling in ffmpeg_parse_infos function, outputting the result from ffmpeg along with the traceback of the unhandled exception.

[coveralls]

Coverage increased (+0.02%) to 67.175% when pulling e9fac98 on mondeja:ffmpeg_parse_infos-tb into 428b46c on Zulko:master.