Systematic error testing

[Debilski]

Moving some of the test files to external Python files and modules (instead of creating them on the fly in a temporary folder) to allow for easier testing on the CLI.

Fixed the problem when in the UI only errors during loading would be shown. We now also show the reason for an error during the game itself.

Unfortunately, testing for error strings is difficult as this changes between Python versions. All in all, we will need to improve the way we do error handling.

[codecov-commenter]

Codecov Report

Merging #765 (d1c6abb) into main (0176b15) will increase coverage by 0.14%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #765      +/-   ##
==========================================
+ Coverage   85.70%   85.85%   +0.14%     
==========================================
  Files          21       21              
  Lines        2351     2354       +3     
==========================================
+ Hits         2015     2021       +6     
+ Misses        336      333       -3     

Files Changed	Coverage Δ
pelita/game.py	86.50% <100.00%> (+0.03%)	⬆️
pelita/network.py	59.79% <100.00%> (+1.98%)	⬆️
pelita/team.py	92.76% <100.00%> (ø)