nsIRequestObserver
Called to signify the beginning of an asynchronous request.
An exception thrown from onStartRequest has the side-effect of
causing the request to be canceled.
| aRequest | request being observed |
| aContext | user defined context |
Called to signify the end of an asynchronous request. This
call is always preceded by a call to onStartRequest.
An exception thrown from onStopRequest is generally ignored.
| aRequest | request being observed |
| aContext | user defined context |
| aStatusCode | reason for stopping (NS_OK if completed successfully) |