Expose error messages to localization or convert all messages to int enum codes

[paulshapiro]

For example, monero_transfer_utils has quite a few errors that should be localized. They're also generally returned as error codes (and should be returned as such where they are not currently) so it may be better to skip localization of strings in this repo itself and do it on the app level.

However many strings here are likely to be shared with Monero source as well (such as on errors while decoding mnemonics), so this is partially also an issue to expose those Monero strings to localization as well, in some sustainable and hopefully upstream-contributable manner.