handle request errors
Some requests to the window server may generate an error.
For example, if a window is destroyed while it is being repositioned
the request to place the window in the new position will fail.
Normally, errors cause a process to terminate.
The ReqErr class is used to handle errors within an application.
The base class defines the attributes associated with each error,
including a message string describing the error,
an integer request code that specifies the kind of call that
generated the error, and a id object that was the target
of the request.
When an error occurs, the attributes of the current request handler
are assigned and the handler's Error operation is called.
- ReqErr* Install()
Use this error handler for subsequent errors and return
the previous error handler.
- virtual void Error()
Handle a detected error.
The default handler is a nop.