We follow standard REST API Error codes to indicate what could be wrong.

HTTP 400: Bad Request.
HTTP 401: Invalid Admin API Key.
HTTP 402: Payment Required.
HTTP 403: Role is insufficient. Example: The Admin API key has view access and is trying to edit.
HTTP 404: The resource does not exists or the Admin API key has no access. For example, the role does not have view or edit permissions on the resource.
HTTP 409: Conflict. For example, the resource already exists.
HTTP 429: Rate limited. Rate Limiting for more info.