[Preferences] Ability to Cancel the OnSave event

[DamianSuess]

As a user, I want the ability to forcibly cancel the OnSave event for the Preferences window to inform the user of an invalid setting which they are attempting to save.

[DamianSuess]

Consider:

• Using, BasePreferencePageActions which uses the interface, IPreferencePageActions for all preference pages to inherit
• Displaying warning message "in the form" rather than a MsgBox