Improve `crop_gribs --backfill` completion log message

[douglatornell]

The log messages at the completion of the crop_gribs worker with the --backfill option has been improved from referring to a specific file to referring to the directory tree containing all the cropped files.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.65%. Comparing base (d9f5fbf) to head (a4a827c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #279 +/- ## ======================================= Coverage 77.65% 77.65% ======================================= Files 133 133 Lines 18695 18695 Branches 1910 1910 ======================================= Hits 14517 14517 Misses 4111 4111 Partials 67 67 ``` | [Flag](https://app.codecov.io/gh/SalishSeaCast/SalishSeaNowcast/pull/279/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SalishSeaCast) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/SalishSeaCast/SalishSeaNowcast/pull/279/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SalishSeaCast) | `77.65% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SalishSeaCast#carryforward-flags-in-the-pull-request-comment) to find out more.

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