You can always go to System Profile => Agent Group => Sensor Configuration
You have to edit .NET Exception or Java Exeption sensor, and add exclude rule for this request. But they will rekord as HTTP 500 as well because this is code returned by WebServer. As I understand, you want to not count this error as failure rate, right? Actually Error Detection rules allows only adding extra ones, but not excludes. Try what I've suggest, and tell me about results.