Show Error Detail for 500 internal server error

Web.Config

<customErrors mode="off"/>

<system.webServer>      
        <httpErrors errorMode="Detailed" /> 
        <asp scriptErrorSentToBrowser="true"/>    
    <defaultDocument> 
      <files> 
        <remove value="default.aspx" /> 
        <add value="default.aspx" /> 
      </files> 
    </defaultDocument> 
</system.webServer>