fix error message for missing cols in mapping df

[codecov-commenter]

Codecov Report

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

Project coverage is 72.79%. Comparing base (a097ee9) to head (f8e2fde).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #323 +/- ## ======================================== Coverage 72.79% 72.79% ======================================== Files 52 52 Lines 4907 4907 Branches 841 841 ======================================== Hits 3572 3572 Misses 1028 1028 Partials 307 307 ```

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