The actual implementation still relies on file paths, but with the
abstraction layer in place, an FS implementation can ensure atomic file
accesses in the future.
Previously, NodeFromFileInfo used the original file path to create the
node, which also meant that extended metadata was read from there
instead of within the vss snapshot.
We don't need `error` here: the only existing implementation
of `ErrorHandler` always call `Backup.Error` and all
implementations of `Backup.Error` always return nil.
The VSS support works for 32 and 64-bit windows, this includes a check that
the restic version matches the OS architecture as required by VSS. The backup
operation will fail the user has not sufficient permissions to use VSS.
Snapshotting volumes also covers mountpoints but skips UNC paths.
