Grird.request; Allow the onLoad to parse and transform the data and add an onError 

The onLoad handler may have already parsed the data and did some transformations.
We don't need to parse it again hence the check if typeof data == 'object'.

Added an event "onError" called when the server returns an error

I think the onError would be better because it allows the program to deal with it its own way.
Not to mention that it removes the (forgotten) dependency with w2ui popup ;)