Hey Navid,
Have you tried using HTTP monitor by reverting an expression on your Data source? this would allow you to check a page only for when the item will appear uppon being checked. In other words, under normal conditions, the page will not return a code 500 or in this case for text "this website is down" but if you are checking for a reverted expression in the datasource of httpmonitor, then you can use that to alert you if this mentioned text appears on the site.
Or in your case, you can check for you web page error such as 500...300 or whatever else.
I hope this helps,
-J