Fix/export xlsx merge cells and write errors

[MartinKl]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 91.22807% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.43%. Comparing base (7c523a6) to head (5aee9ae).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/exporter/xlsx.rs	90.19%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
+ Coverage   84.41%   84.43%   +0.02%     
==========================================
  Files          66       66              
  Lines       24199    24237      +38     
  Branches    24199    24237      +38     
==========================================
+ Hits        20427    20465      +38     
+ Misses       1471     1470       -1     
- Partials     2301     2302       +1     

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