Bugfix: Improve errors for irregular files on Windows

Since Go 1.21, most filesystem reparse points on Windows are considered to be
irregular files. This caused restic to show an `error: invalid node type ""`
error message for those files.

This error message has now been improved and includes the relevant file path:
`error: nodeFromFileInfo path/to/file: unsupported file type "irregular"`.
As irregular files are not required to behave like regular files, it is not
possible to provide a generic way to back up those files.

https://github.com/restic/restic/issues/4560
https://github.com/restic/restic/pull/4620
https://forum.restic.net/t/windows-backup-error-invalid-node-type/6875
