Add support for external modbus devices

[7DM7]

No description provided.

[sourceperl]

Hi,

I don't quite understand the purpose of this merge request. What problem are you trying to solve with this code ? Because, essentially, socket.connect and socket.connect_ex do the same thing, but differently. connect throws an exception in case of an error, and ModbusClient expects this behavior. Whereas connect_ex returns an error code without raising any exception.