We have refactored the error management system to give more visibility to API users.
The message attribute for errors has been replaced with code and error_details, which provide more information about the errors. There are several codes associated with each error.
For example, when creating a new billable metric, the API may return "error": "Unprocessable entity" with the code value_is_mandatory for the name field (i.e. the name of the billable metric is missing).
Please refer to the documentation available for each endpoint for more information on error codes.