Motivation & Background

An error in constructing or modifying a kms_request_t can be obtained by calling:

KMS_MSG_EXPORT (const char *)
kms_request_get_error (kms_request_t *request);

An error in constructing or using a kms_response_parser_t can be obtained by calling:

KMS_MSG_EXPORT (const char *)
kms_response_parser_error (kms_response_parser_t *parser);

There is no API to obtain error messages from a kms_response_t.

Error reporting is currently limited to a string. An opaque error type, similar to libmongocrypt's mongocrypt_status_t, permits future extension.

Scope
Add an opaque error type, similar to libmongocrypt's mongocrypt_status_t. It will be used in new API introduced in KMIP support.