Typing; a few code simplifications

[soininen]

Removed some unused fields from exceptions; added typing to import_mappings.

Checklist before merging

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.07%. Comparing base (a9b947c) to head (2578e6b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
+ Coverage   86.05%   86.07%   +0.01%     
==========================================
  Files          83       83              
  Lines       11433    11447      +14     
  Branches     1635     1635              
==========================================
+ Hits         9839     9853      +14     
  Misses       1241     1241              
  Partials      353      353              

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