Review Classify Errors

[jacobson3]

The modbus module classifies 658400 and 658401 as trivial.

I could not find where these errors could be generated or what they indicate (searched through Modbus Module and Modbus Library). If these are real errors, it would be good to add an Errors section to the README documentation explaining what the errors are and how they are classified. If these are not defined errors we should probably just remove them.