Handle warnings about invalid units for iris>=3.8

[schlunma]

Description

Iris changed the warning message if invalid units are encountered during loading. This PR adds the corresponding pattern to fix this.

Addresses parts of #2376

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[codecov[bot]]

Codecov Report

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

Project coverage is 94.07%. Comparing base (4615da7) to head (b1008c3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2378 +/- ## ======================================= Coverage 94.07% 94.07% ======================================= Files 241 241 Lines 13420 13422 +2 ======================================= + Hits 12625 12627 +2 Misses 795 795 ```

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