Corrected Weight Class Initialization of Artillery Units

[IllianiCBT]

• Scenario generation was treated artillery formations as if they did not use weight classes, resulting in generation failing due to an illegal weight class of -1.
• Changed artillery formations to now be treated like any other formation, allowing for unit substitution in instances where generation fails.
• Changed generation failure messaging to cite weight class integer, not name. This means that if we ever try passing an illegal weight class we will be able to see exactly what was passed, rather than getting an IllegalArgumentException

Fix #5694

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.03%. Comparing base (4cf3be1) to head (4ba3832).
Report is 79 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5701   +/-   ##
=========================================
  Coverage     10.02%   10.03%           
- Complexity     6068     6071    +3     
=========================================
  Files          1074     1074           
  Lines        141641   141640    -1     
  Branches      20645    20643    -2     
=========================================
+ Hits          14206    14209    +3     
+ Misses       126063   126058    -5     
- Partials       1372     1373    +1     

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[Sleet01]

LGTM