Improve feature level error logging when sending files
Improve feature level error logging when sending files

Refs #70

rugk/threema-web,threema-ch/threema-web,threema-ch/threema-web,rugk/threema-web,threema-ch/threema-web,rugk/threema-web,rugk/threema-web,threema-ch/threema-web,threema-ch/threema-web