* Display all errors in showForm() instead of only the first one

* Display all errors in showForm() instead of only the first one
* First do some checks and then delete the target page instead of the opposite so we don't delete a page for no purpose if a problem arises
* Changed some empty() checks to count()
* Use Title::quickUserCan() to see whether to show the checkbox to delete the page or not instead of User::isAllowed()