Refactor: Improve country code conversion failure modes

[hoffie]

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 #2829 (comment)

[pljones]

@hoffie is this likely to get a PR?

[pljones]

Dropping from 3.10.0.