Improve melt error message for invalid measure.vars (#6512)

[ayushsingh9720]

Summary

This Pull Request addresses issue #6512 by making the error message in melt.data.table more informative when a user provides column names in measure.vars that do not exist in the dataset.

Changes

• R/fmelt.R: Added a pre-check for character measure.vars. It uses setdiff() to identify exactly which variables are missing from the data and reports them specifically.
• inst/tests/tests.Rraw: Added a new test case (ID 2154.1) to verify that the specific error message is triggered correctly.
• src/fmelt.c: Cleaned up the C logic to ensure it remains a robust fallback, while the primary informative error is handled in R.

Impact

Instead of a generic:
Error: One or more values in 'measure.vars' is invalid.

Users will now see:
Error: One or more values in 'measure.vars' is invalid; please fix by removing [z1]

This change improves the developer experience by pinpointing the exact typo or missing column in the function call.

Closes #6512

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (291a711) to head (4a82e90).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7549   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files          87       87           
  Lines       16733    16738    +5     
=======================================
+ Hits        16561    16566    +5     
  Misses        172      172           

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