KeyNotFoundException in GetWarningsToBeLoggedAsErrorsByProject

[KirillOsenkov]

msbuild/src/Build/BackEnd/Components/Logging/LoggingService.cs

Line 544 in 80b7f2d

HashSet<string> warningsAsErrorsExcludingMessages = new HashSet<string>(_warningsAsErrorsByProject[key]);

the field has a single entry with a different key.

This is during a pretty regular multi-proc build

[benvillalobos]

This will be closed by #6264