GameReports: remove error message and deal with missing rounds

[SJuliez]

This removes the unnecessary error message for getting a non-existent round report and instead just returns an empty list.

Since the round reports are just a list, not a map of round->reports, they are brittle. But as long as every round gets a report, it works.

This is one of the issues in #5833

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.01%. Comparing base (d349884) to head (12a525f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5836   +/-   ##
=========================================
  Coverage     29.01%   29.01%           
- Complexity    13923    13927    +4     
=========================================
  Files          2511     2511           
  Lines        267117   267114    -3     
  Branches      47784    47784           
=========================================
+ Hits          77504    77508    +4     
+ Misses       185661   185658    -3     
+ Partials       3952     3948    -4     

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