This code system http://terminology.hl7.org/CodeSystem/subscription-error defines the following codes:

CodeDisplayDefinition
dns-resolution-error DNS resolution errorDNS resolution error for client rest-hook endpoint.
no-response No responseNo response for client rest-hook endpoint.
error-response Error responseError response from client rest-hook endpoint. Recommended practice: convey the actual HTTP error in an adjacent Coding or in text
 <status value="draft"/> <experimental value="false"/> <date value="2021-12-02T20:41:54-07:00"/> <publisher value="HL7 (FHIR Project)"/> <contact> <telecom> <system value="url"/> <value value="http://hl7.org/fhir"/> </telecom> <telecom> <system value="email"/> <value value="fhir@lists.hl7.org"/> </telecom> </contact> <contact> <telecom> <system value="url"/> <value value="http://hl7.org/fhir"/> </telecom> <telecom> <system value="email"/> <value value="fhir@lists.hl7.org"/> </telecom> </contact> <description value="Codes to represent subscription error details"/> <caseSensitive value="true"/> <valueSet value="http://terminology.hl7.org/ValueSet/subscription-error"/> <content value="complete"/> <concept> <code value="dns-resolution-error"/> <display value="DNS resolution error"/> <definition value="DNS resolution error for client rest-hook endpoint."/> </concept> <concept> <code value="no-response"/> <display value="No response"/> <definition value="No response for client rest-hook endpoint."/> </concept> <concept> <code value="error-response"/> <display value="Error response"/> <definition value="Error response from client rest-hook endpoint. Recommended practice: convey the actual HTTP error in an adjacent Coding or in text"/> </concept> </CodeSystem>