Check and inform about the configuration (mis)match on saving settings

Problem/Motivation

Actually the settings are independent from Role Paywall, but the Article Test only works for the bundles with Role Paywall applied. So is possible to set up a configuration of the flag that don't work with the configuration of the article paywall

Proposed resolution

Implement a form validation for the settings form, with:

• Check the bundles of the Role Paywall are flagable by the selected flag.
• Check that the flag settings are correct for the expected behavior.
• Set an error if no Role Paywall bundle is flagable byt the selected flag.
• Set a message to inform the user about the setting (mis)match.