<customErrorsmode="On"defaultRedirect="Error.aspx">
  <errorstatusCode="403"redirect="NoAccess.htm"/>
  <errorstatusCode="404"redirect="FileNotFound.htm"/>
</customErrors>

and put [HandleError] above my Controller Class like this.

[HandleError]
        publicclassDSWebsiteController : Controller
        {
            [snip]
            publicActionResult CrashTest()
            {
                thrownewException("Oh Noes!");
            }
        }

Then I let my controllers inherit from this class and call CrashTest() on them. Visual studio halts at the error and after pressing f5 to continue, I get rerouted to Error.aspx?aspxerrorpath=/sxi.mvc/CrashTest (where sxi is the name of the used controller. Off course the path cannot be found and I get "Server Error in '/' Application." 404.

This site was ported from preview 3 to 5. Everything runs (wasn't that much work to port) except the error handling. When I create a complete new project the error handling seems to work.