Create nice error pages for design.gitlab.com

With https://gitlab.com/gitlab-org/design.gitlab.com/merge_requests/1437 we are going to have a nice nginx container for auto-devops. Ths container could serve custom 40x and 50x error pages. Let's create them!

The following discussion from !1437 (merged) should be addressed:

• @leipert started a discussion: (+1 comment)

  @DylanGriffith I am not too familiar with AutoDevOps (shame on me). The built nginx image is run at the end, right? Because if so, we could create a follow-up issue to also provide a nice 40x error template (additionally to the 40x one)