This is a bit hackish. The best way would simply to somehow
use Mastodon's ApplicationController with `use_pack 'error'` from
the Rake task, but I'm not sure how to do that.

- 500.html generated with admin-set default locale if set
- Error page `<title>` includes Mastodon site title
- 500 title changed to "This page is not
  correct" (ref: <https://www.youtube.com/watch?v=2VCAP_seh1A>)
- 500 content appended with "on our end" to make clear it's
  not user's fault

* add haml_lint to Gemfile

* add .haml-lint.yml

* fix warnings of haml_lint

used from nginx when Rails fails.