Localizes any UnifiedError/PayFanoutError for display: the locale's message
for its code when one is registered (resolved per code — exact locale, then
primary subtag, same chain as getUserMessage), otherwise the error's own
(English, user-safe) message. For English (or no locale) the error's own
message wins — it can be more specific than the generic catalog entry.
Localizes any UnifiedError/PayFanoutError for display: the locale's message for its code when one is registered (resolved per code — exact locale, then primary subtag, same chain as
getUserMessage), otherwise the error's own (English, user-safe) message. For English (or no locale) the error's own message wins — it can be more specific than the generic catalog entry.