Add detection for invalid CLI configuration for settings page

This change will log all failures that prevent the CLI cronjob from happening to the database and display a warning to administrators when an error happened.

To test:

1. Configure some invalid CLI php.ini settings
2. Enable the CLI cronjob and run php cron.php
3. See the errors printed and also in the admin page
4. Configure the CLI settings correctly
5. Errors should be gone.

Fixes https://github.com/owncloud/core/issues/13994