Bugfix: Ignore disappeared backup source files

The `backup` command now quietly skips files that are removed between directory
listing and backup, instead of printing errors like:

```
error: lstat /some/file/name: no such file or directory
```

https://github.com/restic/restic/issues/2165
https://github.com/restic/restic/issues/3098
https://github.com/restic/restic/pull/5143
https://github.com/restic/restic/pull/5145
