mars2grib: fix exception lookup in referencePeriod

[MircoValentiniECMWF]

Description

Fix exception lookup bug on mars2grib

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.22%. Comparing base (20a0978) to head (6cde183).
⚠️ Report is 44 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #231      +/-   ##
===========================================
- Coverage    55.41%   53.22%   -2.20%     
===========================================
  Files          322      339      +17     
  Lines        13792    14634     +842     
  Branches      1102     1158      +56     
===========================================
+ Hits          7643     7789     +146     
- Misses        6149     6845     +696     

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