Raising ModbusException in modbus.Slave.handle_request doesn't work

[ErikTempelaarVO]

To test the robustness of our modbus client we'd like the server to throw some errors on requests.

Attempting to do so via hook "modbus.Slave.handle_request" doesn't work properly, because in the except-clause the function code is expected to be available, but it isn't.

modbus-tk/modbus_tk/modbus.py

Lines 713 to 718 in c168f71

	retval = call_hooks("modbus.Slave.handle_request", (self, request_pdu))
	if retval is not None:
	return retval
	# get the function code
	(function_code, ) = struct.unpack(">B", request_pdu[0:1])

modbus-tk/modbus_tk/modbus.py

Lines 745 to 749 in c168f71

	except ModbusError as excpt:
	LOGGER.debug(str(excpt))
	call_hooks("modbus.Slave.on_exception", (self, function_code, excpt))
	return struct.pack(">BB", function_code+128, excpt.get_exception_code())

Fix seems trivial, by reordering the code and fetching the function code before calling said hooks.

Hooking more specifically also works fine:

hooks.install_hook("modbus.Slave.handle_read_input_registers_request", self.on_modbus_slave_handle_request)
hooks.install_hook("modbus.Slave.handle_read_holding_registers_request", self.on_modbus_slave_handle_request)

Does it make sense to request this to be changed? Could open a PR if that's helpful.