Add a function to get errors based on the errorcode

[kevba]

Im the current implementation all the modbuscodes are exposed and can be used if you know which one you want to use. In certain situations, such as a modbus-proxy, the only thing known is an errorcode.

Currently there's no way to get from an errorcode to an error. Goldfish should have a function which allows people to get the correct error based on an errorcode.