Custom Message using iRule for Runtime Error

In my opinion, use a custom page for all types of server-side errors. Server sides errors start with 5xx. Use this irule:

when HTTP_RESPONSE { 
    if { [HTTP::status] > 499 } { 
        HTTP::respond 200 content "Oops Unfortunately, your request casued a error." "Content-Type" "text/html" 
    } 
}