Fix #4078 by refactoring 404.html

DO NOT MERGE YET

I cleaned the HTML code, please say me if there is something wrong here.
Two questions:
 - Can I move the design to a 404-style.css file which will be in the public repository too ?
 - This pull request proposes to insert the two font files and the cat image, do you agree ? Depend of amazon seems really bad for me.

As I do not find how to add new file from github, the images are not linked in this commit, I'll commit them tonight.