Improve error messages related to CMOR tables

[bouweandela]

Description

Closes #issue_number

Link to documentation:

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[codecov]

Codecov Report

❌ Patch coverage is 4.08163% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.40%. Comparing base (3208f5f) to head (45c486d).

Files with missing lines	Patch %	Lines
esmvalcore/dataset.py	4.08%	47 Missing ⚠️

❌ Your patch check has failed because the patch coverage (4.08%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2988      +/-   ##
==========================================
- Coverage   95.68%   95.40%   -0.29%     
==========================================
  Files         267      267              
  Lines       15726    15775      +49     
==========================================
+ Hits        15048    15050       +2     
- Misses        678      725      +47     

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