libcurl's WRITEFUNCTION is pretty fragile; if we destroy the CURL*
instance or even unregister it using curl_multi_remove_handle(),
libcurl will crash instantly. But still we need to be able to handle
exceptions from inside the WRITEFUNCTION, and call
CurlResponseHandler::OnError(), which may destroy the whole thing. As
a workaround, I use DeferredMonitor to postpone the OnError() call
into a stack frame which is allowed to destroy the request.
Convert assertion to runtime check. This is useful because this is a
public method, and the caller has no chance to check if the object is
still registered.
