Ensure the player name in the step definition is valid

[trevan]

Fixes #8214

Testing

I started a game using the map from #8214 and verified the error was shown correctly.
I started a game from a few other maps and verified that an error didn't occur.

Screens Shots

Additional Notes to Reviewer

It is possible that other released maps might show this error. I didn't check every map. But when those errors come in, they should be easily fixed.

Release Note

CHANGE|Invalid player names in a step definition will now cause an error to be shown when the map is first loaded.

[codecov]

Codecov Report

Merging #8384 (a0f89a8) into master (a7a7576) will increase coverage by 0.04%.
The diff coverage is 50.70%.

@@             Coverage Diff              @@
##             master    #8384      +/-   ##
============================================
+ Coverage     24.83%   24.87%   +0.04%     
- Complexity     7304     7341      +37     
============================================
  Files          1265     1274       +9     
  Lines         79592    79520      -72     
  Branches      11085    11034      -51     
============================================
+ Hits          19765    19779      +14     
+ Misses        57754    57682      -72     
+ Partials       2073     2059      -14     

Impacted Files	Coverage Δ	Complexity Δ
.../strategy/engine/data/BombingUnitDamageChange.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...ava/games/strategy/engine/data/UnitHitsChange.java	0.00% <ø> (-70.00%)	0.00 <0.00> (-6.00)
...a/changefactory/units/BombingUnitDamageChange.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...es/strategy/engine/data/gameparser/GameParser.java	81.28% <0.00%> (-0.59%)	142.00 <0.00> (+1.00)	⬇️
...trategy/engine/delegate/DefaultDelegateBridge.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../games/strategy/engine/framework/AbstractGame.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...va/games/strategy/engine/framework/ClientGame.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...va/games/strategy/engine/framework/GameRunner.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...va/games/strategy/engine/framework/ServerGame.java	0.57% <0.00%> (-0.01%)	1.00 <0.00> (ø)
.../system/loader/AvailableGamesFileSystemReader.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
... and 127 more

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