Our current script that downloads the latest revision from the wikipedia.de banner repository fails silently. It should be more verbose and produce actual output in error cases that will get sent to the fundraising team

Implementation Details
Change the script server-setup/files/scripts/update_banners.sh as follows:

• capture both output to stdout and stderr of the git commands. The curly brace syntax currently only captures stdout
• If there is output to stderr, echo it. Since the script is run by a cron job that expects the script to be silent, any output will automatically trigger an email to the fundraising team
• If stderr is empty, log stdout to the log file, prepended by a time stamp
• After you have improved the script (and user-tested it in a local environment, e.g. a docker container), notify the internal IT to run the server-setup/banner_server.yml playbook to the deploy the script.