Data Source Section: Fixed a bug when the group param is a field that got deleted

I just ran into this bug. I deleted a field that was used for DS grouping, but the pages that used this DS crashed with the ugly php error page.

I only added a simple check to see that the group field was found. If not, I throw a `new SymphonyErrorPage`. (class that MUST be renamed by the way). Since the `execute` function is called in a try {} catch () {}, the error page does not show up. Instead, the page load normally and the DS outputs the error in the xml in the error tag, which is pretty neat BTW.