Pass request status to all callback invocations, do not merge changes on error

When saving data on server, the changes should not be merged, if the save
request was not successful.

I have prepared an example at: http://sw-amt.ws/w2ui/demos/#!grid/grid-30
Just edit some fields and click "Save". When an error occurs, the changes are not lost.
Merge pull request #465 from wolfmanx/feat-request-status

Pass request status to all callback invocations, do not merge changes on...