R – Cannot catch the SharePoint Access denied error

I am trying to access a sharepoint list programmatically in a webpart, like this.

try
{
      masterList = web.Lists[listId];
}
catch(Exception e)
{
      RenderExceptionMessage(e.Message);
}

The RenderExceptionMessage() method is supposed to show a user-friendly error message inside the webpart.

But the problem is that I am not able to trap the Exception. Instead the webpart page redirects to an access denied page which shows an error message "You are currently signed in as: Domain\user"

Also, the message of the exception being caught reads "Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack."

Any idea why this behaves this way?