Code should validate relative path in workspace file and behave more gracefully

[dbaeumer]

Testing: #33278

• create a workspace file with only relative paths
• copy the file using OS means (file explorer) where the folder don't make any sense
• open the file

Observe:

• you see the folder structure also not existing on disk
• clicking on a folder shows an error message:

I would have expected that the folders are rendered gray / disabled and that clicking on them shows me a better error message

[dbaeumer]

I would also expect that Remove Folder is available in the context menu

[bpasero]

This basically is #29738

We should still show the root folder without error and allow to remove it. But it should be clear that the folder cannot be found (e.g. greyed out or with different icon). Also, file actions need to be restricted for that case.

[isidorn]

Dup of #29738