The script already won't work without them; so if the user gets the
invocation wrong, give a halfway-reasonable error rather than just
crash into the ground.
This allows the installer to continue using this script for the
`standalone` method, while the no-argument form now uses the same
`webroot` method as the renewal cron job, suitable for running
by hand to adopt Certbot after initial install.
This causes the cron job to run only when a Zulip-managed certbot
install is actually set up.
Inside `install`, zulip.conf doesn't yet exist when we run
setup-certbot, so we write the setting later. But we also give
setup-certbot the ability to write the setting itself, so that we
can recommend it in instructions for adopting certbot in an
existing Zulip installation.
This helps make this script suitable to run on existing installations,
by mitigating any worry about clobbering existing certs with links to
the new ones, in case the admin changes their mind or was using the
certs for something else too.
Greg Price
Tim Abbott
rht