Bugfix: Ignore disappeared backup source files

If during a backup files were removed between restic listing the directory
content and backing up the file in question, the following error could occur:

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

The backup command now ignores this particular error and silently skips the
removed file.

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