When the config file is not writable, and the config file is written to disk, it currently silently fails. All writes to the config file are done on _destruct which means we can't display an error message to the screen when it fails.
Instead, on the screens that require a writable config file, we should display a warning message that the config file is not writable.
The screens affected are:
(In ) Fixes #1153 Adding warning message in UI if config file not writable for the super user "Settings" screen, and in the "Plugins" screen
(In ) Refs #1153 adding warnign for general settings screen as SMTP config requires config file writable