Error pages

Update web.config to use CMS error pages.

<customErrors mode="On">
  <error statusCode="404" redirect="~/404/" />
  <error statusCode="500" redirect="~/500/" />
</customErrors>

Note: in VisualSutio 2012 MVC3 application project by default there is registered global action filter HandleErrorAttribute, which renders ~/Views/Shared/Error.cshtml on error. If you want to use customErrors redirects from web.config - you need not to register HandleErrorAttribute filter.

If you want to configure CMS to use your custom 404 error page - update cms.config.

<cms pageNotFoundUrl="/404/">[...]</cms>