Allow missing files in `sync-files --latest`
Allow missing files in `sync-files --latest`

When running `sync-files` (or `sync-identical-files`) with the `--latest` switch, if one or more of the files in a group does not exist, the script will crash. This happens all the time when I add a new group, or add a new file path in an existing group. This has bothered me for a long time, so I finally fixed it when I ran into it again today.

I've changed the script as follows:
- If _none_ of the paths in the group exist, print an error message listing the paths in the group. This happens with or without `--latest`.
- If `--latest` is specified, copy the master file to the paths of the missing files.

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