[fix] debconf: avoid infinite loop (#1517)

Don't loop when debconf frontend is non-interactive.
[fix] Allow to use a remote sql server even if a local one is installed
[fix] debconf: don't fail when noninteractive frontend is used
[enh] remote mysql: show an error when connection fails
[fix] debconf: use error instead of note (#1524)

When encountered, errors are always showed.