add task and query for finding mismatched task.status != task.results.status
patch from @kookster to address one case of task.status mismatch #575
update nil duration based on completed transcoding tasks
set audio_file.duration based on transcoded length, if duration is not already set
fix method name typo; add debugging line to otherwise no-op abstract method
gah. "else" always triggers. bare "rescue" does not
zap redundant worker process (follow on to 7c5d310 #575)