Add default error handler

Add default error handler to CMS. If production mode is enabled and cms.errorhandler.redirect config value is not empty CMS would redirect user from the error page to the one set into config. This would help to provide sites static page that won't be affected by any possible runtime errors (e.g. lost db connection, exception in block).

This change won't affect exiting CMS setups since by default this handler do just the same as the default one does.