Refactor: Improve country code conversion failure modes

> I'd rather "convert to wireformat" and "convert from wireformat" were both "safe" - i.e. returned "Error" (e.g. -1) or a valid response, given any integer value, then have the caller deal with the error as it sees fit. Having to "know" about the `IsCountryCodeSupported` on top of the conversion methods seems excessively complicated.

_Originally posted by @pljones in https://github.com/jamulussoftware/jamulus/pull/2829#discussion_r965319280_