Improve MKCOL error handling + potential security fix

[eskimor]

Description

Sync with easysync does not work at all. It gets stuck with no further info, I did see a few error messages on the server which indicates an improper implementation of the WebDAV protocol. I fixed those and also I believe a security issue, which would allow users to create folders in folders of other users. easysync still does not work though, nevertheless filing this PR as I think it improves the status quo. I am not 100% happy, in particular passing an Option, which is always Some in production code seems unnecessary, but fixing this would be quite a bit of a refactor, which seems out of scope for a first-time contribution.

In particular, the server responds with internal server error, instead of the METHOD_NOT_ALLOWED when creating a directory that already exists.

Type of Change

Please check the option that best describes your change:

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I compiled & ran it on my server.

Checklist:

• My code follows the style guidelines of this project
• 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 - actual fix is hard to test right now, would need some separate cleanup, which I would happily do if welcome.
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules