Improve handling of numeric codes and commands

[avojak]

Rather than using a bunch of static consts for the numerics and commands it would probably be better to use enum(s) which can hold the numeric or string value as a property. This would also allow us to group by error/non-error and have a default handler rather than needing to specifically list cases for each (e.g. we can handle cases specifically when it makes sense for better formatting, but have a default to display error/message content rather than silently dropping messages).