refactor: share package registry error status classification (#39133)

Signed-off-by: silverwind <me@silverwind.io>

routers/api/packages/helper/helper.go

@@ -4,6 +4,7 @@
 package helper
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -12,9 +13,18 @@ import (
 	packages_model "gitea.dev/models/packages"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
+	"gitea.dev/modules/util"
 	"gitea.dev/services/context"
 )
 
+// PackageErrorStatus returns the status to report for a package lookup error
+func PackageErrorStatus(err error) int {
+	if errors.Is(err, util.ErrNotExist) {
+		return http.StatusNotFound
+	}
+	return http.StatusInternalServerError
+}
+
 // ProcessErrorForUser logs the error and returns a user-error message for the end user.
 // If the status is http.StatusInternalServerError, the message is stripped for non-admin users in production.
 func ProcessErrorForUser(ctx *context.Context, status int, errObj any) string {