Bugfix: Skip inaccessible `backup` source paths

The `backup` command only skipped source paths that did not exist. A path that
could not be accessed for another reason, such as a malformed path on Windows,
was kept and produced an empty snapshot. Restic now skips any such path and
aborts if none remain.

https://github.com/restic/restic/issues/5667
https://github.com/restic/restic/pull/21852
