better error msg for missing GPG

Ups, something went really wrong here. It seems GPG is missing on your
system. Please install it. Find it e.g. in the GPGTools Installer on
https://www.gpgtools.org.
-> GPG_NOT_FOUND_MESSAGE = "Ups, something went really wrong. It seems
GPG is missing on your system. Please reinstall GPG Suite from
https://www.gpgtools.org.