Fix importer fewer columns than expected Traceback

[PiispaH]

Now when an importer spec is modified to have a column reference that is out of range for the source data, an error is shown. If such importer spec is executed an error will also be thrown and logged.

Fixes spine-tools/Spine-Toolbox#2333

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes in Toolbox repo have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• Unit tests pass

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (90c8780) 55.46% compared to head (d1e89da) 55.43%.
Report is 1 commits behind head on 0.8-dev.

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Additional details and impacted files

@@             Coverage Diff             @@
##           0.8-dev     #160      +/-   ##
===========================================
- Coverage    55.46%   55.43%   -0.03%     
===========================================
  Files          192      192              
  Lines        17286    17306      +20     
  Branches      2829     2832       +3     
===========================================
+ Hits          9587     9593       +6     
- Misses        7179     7192      +13     
- Partials       520      521       +1     

Files	Coverage Δ
spine_items/importer/mvcmodels/mappings_model.py	53.49% <25.00%> (-0.15%)	⬇️
spine_items/importer/widgets/import_sources.py	54.34% <36.84%> (-1.05%)	⬇️

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