Modbus: exception 0x5

[Mark81]

When reading from a remote device, sometimes I get the exception 0x5 (request timeout). The documentation says:

Specialized use in conjunction with programming
commands.
The server has accepted the request and is
processing it, but a long duration of time will be
required to do so. This response is returned to
prevent a timeout error from occurring in the
client. The client can next issue a Poll Program
Complete message to determine if processing is
completed.

But I cannot find this function in QModbus* classes.
How should I handle this exception?