Make the pretty 404s display for the errors and make a 400 error page

- Have to use layout: "error_page" otherwise it doesn't render without
  the login part of the heading.
- Reindent some of the return statements.
Re-add "status: xxx" into the render line
Add a trailing new line to error_400.html.erb
Refactor the error rendering lines into one to avoid repetition
Merge pull request #200 from alphagov/use_graphical_errors_in_mappings_find_global

On error, display pretty pages for 400 and 404 errors