Consider returning HTTP 500 for feed URLs

[JakeGinnivan]

From @mabster

When a database error occurs, we redirect to the admin login page with a databaseIssue=true URL parameter (line 27 in /src/FunnelWeb.Web/Application/RequireUpdatedDatabaseHttpModule.cs).
This is not useful when requesting a feed URL.

I suggest we add a special case to the method which tests if the request if calling a feed URL ("/feed" and "/commentFeed") and simply return an HTTP 500 or 503 to readers.