when config file is not writable, display a warning message

[matt]

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:

• Plugins management: plugin enable/disable
• Super user password reset
• Super user email change

[matt]

(In [2183]) Fixes #1153 Adding warning message in UI if config file not writable for the super user "Settings" screen, and in the "Plugins" screen

[matt]

(In [2664]) Refs #1153 adding warnign for general settings screen as SMTP config requires config file writable