Improve subprocess error handling.
Merge pull request #109 from t-animal/master

Speed up startup and introduce progress bars
pep8
Skip files that don't exist in the ZIP archiving (ref #110).
Add missing label for the progressbar with 1 core.
Remove files that can't be processed for some reason (fix #112).
Add failed files to the cli summary.
Move Devnull to the utils module.
Merge branch 'remove-failed-files'
Use cached properties to simplify the access to exif metadata.
Fix unicode no more exist on py3.