Once more with feeling

http://test.archiveofourown.org/tags/4166666666607/feed.atom should return a 404 error not
http://test.archiveofourown.org/tags with a flash message

The works controller should throw a 404 when a work is not found
http://test.ao3.org/works/1848202/chapters/3974776

The following url should be a 404
http://test.ao3.org/users/zz9pzza/pseuds/james_

http://test.ao3.org/admin_posts/99999 should raise a 404 error
Fix typo
really fix that error
Update admin_posts_controller.rb
Update pseuds_controller.rb
Update tags_controller.rb
Be explit about the user we can't find
Double space
Following Ariana's comments
Update pseuds_controller.rb

Couldn't not could not
Remove Sorry
Merge pull request #1776 from zz9pzza/issue_4054_2

Issue 4054: Respond with 404 instead of redirect: admin posts, pseuds, works, tags