Add localization to custom channel event exception messages

Channel manager event handlers (see 1) can set custom user messages, but these are not localizible since the user locale is not available to event handlers. If the call to the channel rest endpoints gets that locale and accepts it, custom messages can be read from the repository and localization can be added.

(1) https://documentation.bloomreach.com/library/concepts/channels/overview-of-channel-manager-events-and-response-mechanisms.html