The warning messages that can appear on the main site (after logging in)
can be added by hooks now, too, so the page.inc.php of the "main" module
doesn't contain code specific to a dozen other modules.

If you want to add a hook, create a file in your module:
./modules/<module>/hooks/main-warning.inc.php
The file contains flat code, no classes or functions. You should
do your checks, and then call Message::addError (or the likes).
You could also render your own template if you feel fancy.
Remember you're running in the context of the main module, so your
message ID should use the full syntax module.id, or add your
module's name to the Render::addTemplate call.

If you also want to enable the "configuration incomplete" global
warning, set $needSetup to true.

See modules "vmstore", "minilinux" or "sysconfig" for an example.