support "code" attribute of error messages

[csmuc]

• codes added to errors as symbols
• can be mapped with i18n by clients

The codes are "taken" or "invalid" and can be mapped to different languages with Rails i18n.

[golcor]

👍
any feedback? this would be quite useful for our project

[senid231]

Hi @csmuc
according to changes in PR #277 you can now just override #error_key and #error_msg for JsonApiClient::ErrorCollector::Error at lib/json_api_client/error_collector.rb:43

I think we should not change current behavior of using title || detail as error message because it may break existing functionality.
The better way would be to allow overriding it easily for application specific purposes

[senid231]

@golcor can you say if overriding of #error_key and #error_msg in JsonApiClient::ErrorCollector::Error provide all that you need for your project, or maybe something is missing?