Use named return values in WriteAtomically

[twz123]

Description

The intention was to use a named return value, so that the deferred function would be able to modify it. Without the named value, the error handling in the deferred function has no effect.

Moreover: Add a bunch of tests and suppress any errors about failed removals of temporary files indicating that the files don't exist.

Fixes #1816.

The tests should've been added in the first PR already, but sometimes we can't help being a little sloppy 🙈

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings