Diagnostics message is always printed to MSBuild log as an "Unknown build error"

[dosvidos]

The message

Unknown build error: Executing command

is always written as a "normal" severity message
It's generated here: https://github.com/tunnelvisionlabs/antlr4cs/blob/master/runtime/CSharp/Antlr4BuildTasks/Antlr4ClassGenerationTaskInternal.cs#L317
It has no error code, so it's get assigned an AC1000 code and the error prefix here: https://github.com/tunnelvisionlabs/antlr4cs/blob/master/runtime/CSharp/Antlr4BuildTasks/Antlr4ClassGenerationTask.cs#L207-L212
And then printed to the build output here: https://github.com/tunnelvisionlabs/antlr4cs/blob/master/runtime/CSharp/Antlr4BuildTasks/Antlr4ClassGenerationTask.cs#L226