fix decode of stderr lines

[chadawagner]

Encountered a case where non-utf8 ID3 tags in normal stderr output will cause exceptions in an otherwise successful process. Telling decode() to ignore or replace errors fixes the issue. This PR uses "backslashreplace" as the codec error handler, rather than the default which is "strict".

Opened https://github.com/jonghwanhyeon/python-ffmpeg/issues/57