Fix AppendOp if the file is unwritable or the containing directory

Problem/Motivation

AppendOp can have the same problems as ReplaceOp as detailed in #3091285: Composer scaffolding fails when permissions on default.settings.yml or default.settings.php is not writable.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet