Refactor Hardcoded Error Messages

Error messages are hardcoded either in exceptions or as return values in implementation. If these are to be displayed to the end user, we should use the language_tag fields we have to localize them. If these are instead handled by other developers and they require more than just the HTTP error code, we should number possible errors and return that error value so developers can switch on how to handle them.

Start by inventorying the implementation's hardcoded messages in exceptions and return values, then inspect how language_tag fields and HTTP error codes are currently handled. Separate messages intended for end users from errors consumed by other developers. Done means the project has a decided, consistently applied approach for localization or numbered error values, with affected API behavior documented.

Written by the indexing model from the issue text.