如何将非阻塞stderr捕获添加到线程popen

I've got a python 3 script i use to backup and encrypt mysqldump files and im having a particular issues with one database that is 67gb after encryption & compression. The mysqldump is outputting errorcode 3, so i'd like to catch the actual error message, as this could mean a couple of things. The random thing is the backup file is the right size, so not sure what the error means. it worked once on this database...I've got a python 3 script i use to backup and