Update 404 handling. Fixes #832.

- Uses webapp2's error handling for 404 errors.
- Changes previous redirects to /error/404 to just throw 404's in place.
Fix RSS template: close double braces
Merge pull request #850 from mc10/rss

Fix RSS template: close double braces
Merge pull request #849 from gregmarra/404

Update 404 handling. Fixes #832.