Fix/conllu dropped column should be an error

[MartinKl]

fixes #325

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.27%. Comparing base (fc6f77d) to head (27b3a48).

Files with missing lines	Patch %	Lines
src/importer/conllu/mod.rs	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
+ Coverage   83.24%   83.27%   +0.02%     
==========================================
  Files          54       54              
  Lines       19846    19836      -10     
  Branches    19846    19836      -10     
==========================================
- Hits        16521    16518       -3     
+ Misses       1199     1192       -7     
  Partials     2126     2126              

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