404.html: better content vertical alignment

Content vertical alignment using `display: table` for `<html>` and `display: table-cell` for `<body>`.
That way one can modify the content and don't worry about readjusting the margin.
Let add `margin: 0 auto` to make the display in IE 6/7 a little bit better.

* `margin: auto;`
  ![](https://cloud.githubusercontent.com/assets/1223565/3763997/13b638ae-18ab-11e4-965c-9117aef40587.png)

* `margin: 0 auto;`

  ![](https://cloud.githubusercontent.com/assets/1223565/3763998/13dad6aa-18ab-11e4-9468-52e81876c518.png)