Error Messages
Error messages can be sent on three different levels.
Errors in StringMessage
Errors on the outer level include cases like unavailable operators or invalid content. They are not version-specific.
Errors in Content/Payload
Errors related to valid requests might be handled "inline" in the matching response if the operator is able to create a response, otherwise an ErrorResponse may be used (<GeneralErrorType> in StringError.xsd).
Errors in Responses
The response types have their own error fields for informational messages and non-fatal errors. See the documentation of the request/response types.