This PR changes the defaults when sources are missing. Previously, a missing source would abort the backup, unless `--allow-missing-source` was set.
With this change, if no options are set, a missing source will only trigger a warning.
If the option `--allow-missing-source` is set, no warning is logged.
To get the previous behavior, a new option `--abort-if-source-missing` is introduced that toggles this.
If no sources are found at all, the backup will abort no matter the flags.
