use bootstrap alerts for 404

[faaez]

http://twitter.github.com/bootstrap/javascript.html#alerts

[shreshth]

Just a note if anyone decides to do this: As of now, all 404s go through 'not_found.html', accessed through exception handlers on the pages.

[zeerakahmed]

This is the best. We can use this for all the other alerts also. Not verified producers etc.