fix: assorted ActivityPub code only refactors (#8708)

Fix parts of issue #8221 and part of PR #4767

Is linked to https://codeberg.org/forgejo/forgejo/pulls/8274

The commit 555f6e57ad fixes timeout forgejo/forgejo#8274 (Kommentar)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8708
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-committed-by: Michael Jerger <michael.jerger@meissa-gmbh.de>

services/federation/error.go

@@ -0,0 +1,44 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package federation
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type ErrNotAcceptable struct {
+	Message string
+}
+
+func NewErrNotAcceptablef(format string, a ...any) ErrNotAcceptable {
+	message := fmt.Sprintf(format, a...)
+	return ErrNotAcceptable{Message: message}
+}
+
+func (err ErrNotAcceptable) Error() string {
+	return fmt.Sprintf("NotAcceptable: %v", err.Message)
+}
+
+type ErrInternal struct {
+	Message string
+}
+
+func NewErrInternalf(format string, a ...any) ErrInternal {
+	message := fmt.Sprintf(format, a...)
+	return ErrInternal{Message: message}
+}
+
+func (err ErrInternal) Error() string {
+	return fmt.Sprintf("InternalServerError: %v", err.Message)
+}
+
+func HTTPStatus(err error) int {
+	switch err.(type) {
+	case ErrNotAcceptable:
+		return http.StatusNotAcceptable
+	default:
+		return http.StatusInternalServerError
+	}
+}