Installer fails if cron is not installed

[B-X-M]

Affected: Debian 10, possibly other distros

The installer fails if the server relies on systemd timers only and thus does not provide the standard cron mechanism (standard in many containerized versions of e.g. debian). Failure message indicates that it cannot write to /etc/cron.d as it does not exist.

Workaround: Make sure cron is installed before running the installer.

Fix: Check that /etc/cron.d is writable, provide useful message to the user.

[almereyda]

As the description shows an issue with a non-existing directory, #81 seems related here.