Warn on missing configuration for data product

[knoepfel]

FORM unconditionally throws whenever a configuration is not found for a particular data product. This PR relaxes that to a warning message for now.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
form/form/form_writer.cpp	0.00%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

@@           Coverage Diff           @@
##             main     #473   +/-   ##
=======================================
  Coverage   85.42%   85.43%           
=======================================
  Files         144      144           
  Lines        3589     3591    +2     
  Branches      615      615           
=======================================
+ Hits         3066     3068    +2     
- Misses        313      315    +2     
+ Partials      210      208    -2     

Flag	Coverage Δ
unittests	85.43% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
form/form/form_writer.cpp	72.41% <0.00%> (-5.37%)	⬇️

... and 3 files with indirect coverage changes

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