Cosmo Error page. It has all you need – links to your other pages, links to your social profiles, working contact form.
Used resources:
Warning Icon. (Freeware, commercial usage: Allowed) http://www.iconarchive.com/show/3d-vol2-icons-by-3dlb/warning-icon.html
jQuery plugin: Validation. (License: MIT /GPL) http://bassistance.de/jquery-plugins/jquery-plugin-validation/
Fancybox
Tipsy