Revert staticPage setting on validation or server error

closes #3134
- reset `page` property in PSM if errors occur after un/setting static page in editor
Do you also need to mess with isDirty sorts of things here? It gets scary -- if this was the only thing that made the post dirty, then it's not actually dirty, but if something else had already made it dirty, it's definitely still dirty..?

Or wait. is isDirty even something we talk about in this context? Since static page gets toggled only here, it doesn't actually dirty-ify does it?