Change: let `backup` store files with incomplete metadata

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

Now, restic also includes items for which the extended metadata could not
be read in a snapshot. The warning message has been changed to read
```
incomplete metadata for /path/to/file: details on error
```

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