refac: missing user or article is reported with a domain not-found error instead of a raw GORM error, and the API answers 404 for it

api/pkg/apperr/messages.go

@@ -3,3 +3,4 @@ package apperr
 const LogsErrorMessage = "Access to the requested resource is forbidden due to incorrect password and/or username."
 const BodyErrorMessage = "The request cannot be processed due to a mismatch in the format of the body."
 const IdUint32ErrorMessage = "The server expects the ID to be in the format of an unsigned 32-bit integer (uint32)."
+const ArticleNotFoundErrorMessage = "Article not found"