This case-sensitive code system http://hl7.org/fhir/response-code defines the following codes:
Code
Display
Definition
ok
OK
The message was accepted and processed without error.
transient-error
Transient Error
Some internal unexpected error occurred - wait and try again. Note - this is usually used for things like database unavailable, which may be expected to resolve, though human intervention may be required.
fatal-error
Fatal Error
The message was rejected because of a problem with the content. There is no point in re-sending without change. The response narrative SHALL describe the issue.