the thumbnailer / audio transcoder could return misleading errors
if the operation fails due to insufficient filesystem permissions
try reading a few bytes from the file and bail early if it fails,
and detect/log unwritable output folders for thumbnails
also fixes http-response to only return svg-formatted errors
if the initial request expects a picture in response, not audio
* add readme section on using amazon/aws s3 as storage
* mention http/https confusion caused by incorrectly configured cloudflare
* improve custom-font notes
* docker: ftp-server howto
* docker: suggest moving hist-folders into the config path
and switch the idp docker-compose files to use the
main image, in anticipation of v1.11