Bug 498890

SUMMARY
I have a local network share that gets mounted on startup by systemd. As I use it for bulk data storage, I have all of my "Locations" paths set to directories on that share. Whenever the server can't be reached during Plasma's startup (network hiccups, maintenance etc.), Plasma will permanently cut off the part of the path that isn't reachable, requiring manual reassignment.

STEPS TO REPRODUCE
1. Configure a network share to be mounted in some directory (like `/home/user/share`) before Plasma starts up (systemd unit with `WantedBy=multi-user.target`)
2. In System Settings > System > Session > Locations, set one or more locations to directories on that share (like `/home/user/share/Desktop`)
3. Reboot and disconnect the network, or make the share mounting process fail in some other way

OBSERVED RESULT
Configured path permanently changes to the furthest it can reach at that moment. For example, `/home/user/share/Desktop` becomes `/home/user`. It remains like that even after another reboot where the mount succeeds, and has to be fixed manually.

EXPECTED RESULT
Configured path should remain the same, but attempting to access the nonexistent directories should fall back on the "cut" version (maybe with a warning message?). The configured paths should not be changed without asking the user.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kernel 6.12.9-arch1-1
KDE Plasma Version: 6.2.5
KDE Frameworks Version: 6.10.0
Qt Version: 6.8.1

ADDITIONAL INFORMATION
My systemd mount file:
```
[Unit]
Description=Mount keepstar/43615
After=nss-lookup.target

[Mount]
What=keepstar:/mnt/sto/43615
Where=/home/aegis/43615
Options=vers=4
Type=nfs
TimeoutSec=90

[Install]
WantedBy=multi-user.target
```