<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<httpErrors errorMode="Detailed">
<remove statusCode="404" subStatusCode="-1" />
<remove statusCode="500" subStatusCode="-1" />
<error statusCode="500" prefixLanguageFilePath="" path="/404.asp" responseMode="ExecuteURL" />
<error statusCode="404" prefixLanguageFilePath="" path="/404.asp" responseMode="ExecuteURL" />
</httpErrors>
</system.webServer>
</configuration>

web config dosyanızı yukarıdaki gibi yapmanız yeterli olacaktır.