overdue: add warning when the config file cannot be found

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
overdue: add missing default config file

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
overdue: trivial cleanups

No functional change.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
overdue: don't run the machinery if the config is a no-op

By default, overdue is disabled. For performance reasons, don't bother
inserting entries in the notification queue in that case, as the overdue state
will never change (clear).

This surfaced a test setup issue in Beatrix and a missing binding in Guice.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>