You can monitor url, when it down multiple time you can get an email alert.
This is free and included.
Select the monitoring button on your product
Scroll down the page to bottom
Change the keyword to search, use a keyword not used in error case. Prefer keyword into the footer. Customise your url if needed. Click add.
Done. You will receive an email from email@example.com when error occur. To be sure receive, whitelist firstname.lastname@example.org into your email provider.