Fix Notify callback calls in go-nakadi.Processor

[krnkl]

when cursor is commited error is not checked and Notify callback is not called:

        ....
		stream.CommitCursor(cursor)
	}

Moreover, error on line
is shadowed an thus not reported to callback notify function:

        err = operation(streamNo, cursor.NakadiStreamID, events)
	if err != nil {
	    err = stream.
            if err != nil {
                   options.NotifyErr(err, 0)
	}