The logging was broken in 958cbf38a44d4092934e51266b5aa88d224df6e6 (DSPTool doesn't use dolphin's logging system, so it just produced nothing; the same thing affected comparing before 693a29f8ceeb1166658d9c0752b6e84660077374).
AssemblerError::LabelAlreadyExists (previously ERR_LABEL_EXISTS) simply was never used.