fix: add missing error properties

Add some properties missing from the JSON API specification, which
v3-go-libs supports, which need to match the schema of error responses.

See https://jsonapi.org/format/#error-objects

components/errors.yaml

@@ -35,6 +35,14 @@ Error:
       type: string
       description: "A human-readable explanation specific to this occurrence of the problem."
       example: "The request was missing these required fields: ..."
+    code:
+      type: string
+      description: "An application-specific error code, expressed as a string value."
+      example: "entity-not-found"
+    title:
+      type: string
+      description: "A short, human-readable summary of the problem that SHOULD NOT change from occurrence to occurrence of the problem, except for purposes of localization."
+      example: "Bad request"
     source:
       type: object
       properties: