'Called on page download start.
@param partialContent: tells us if the download is partial download we requested.'

def pageStart(self, partialContent):

    if (partialContent and (not self.requestedPartial)):
        raise ValueError("we    shouldn't    get    partial    content    response    if    we    didn't    want    it!")
    if self.waiting:
        try:
            if (not self.file):
                self.file = self.openFile(partialContent)
        except IOError:
            self.deferred.errback(Failure())
