Use Hook to add valid error messages

Dependency: I9e996a88e3972f09946726060916a21124de049c
Bug: 71769
Change-Id: I57e6140b1973e5fc4814da88c14224f3fc7e562b
Really Activate $wgUserMediaWikiUIEverywhere everywhere

Set $wgUseMediaWikiUIEverywhere as early as possible (some contents
will be created before Skin is created and will not be MW UI).

Bug: 72853
Change-Id: I18fecda9aeb3e0babd92146d6e6413abf296bed3
Merge "Really Activate $wgUserMediaWikiUIEverywhere everywhere"
Merge "Use Hook to add valid error messages"