Change: `backup` includes files with incomplete metadata

If restic failed to read extended metadata for a file or folder during a
backup, then the file or folder was not included in the resulting snapshot.
Instead, a warning message was printed along with returning exit code 3 once
the backup was finished.

Now, restic also includes items for which the extended metadata could not be
read in a snapshot. The warning message has been updated to:

```
incomplete metadata for /path/to/file: <details about error>
```

https://github.com/restic/restic/issues/4953
https://github.com/restic/restic/pull/4977