user deletions: fix error when given incorrect password.

Use validations instead of raising an exception when the password is
incorrect so that the controller can display errors sensibly.

Also fix users being logged out even when the deletion attempt failed
due to an incorrect password.

app/controllers/maintenance/user/deletions_controller.rb

@@ -1,14 +1,23 @@
 module Maintenance
   module User
     class DeletionsController < ApplicationController
+      respond_to :html, :json, :xml
+
       def show
       end
 
       def destroy
         deletion = UserDeletion.new(CurrentUser.user, params.dig(:user, :password))
         deletion.delete!
-        session.delete(:user_id)
-        redirect_to(posts_path, :notice => "You are now logged out")
+
+        if deletion.errors.none?
+          session.delete(:user_id)
+          flash[:notice] = "Your account has been deactivated"
+          respond_with(deletion, location: posts_path)
+        else
+          flash[:notice] = deletion.errors.full_messages.join("; ")
+          redirect_to maintenance_user_deletion_path
+        end
       end
     end
   end