DSPTool: Fix missing error when redefining labels

[Pokechu22]

The logging was broken in 958cbf3 (DSPTool doesn't use dolphin's logging system, so it just produced nothing; the same thing affected comparing before 693a29f).

AssemblerError::LabelAlreadyExists (previously ERR_LABEL_EXISTS) simply was never used.

[Pokechu22]

For context, I created a loop with a label wait_dma_finish in one of my tests, but it turns out that label is also used by the included dsp_base_noirq. This produced an infinite loop where stuff was pushed to the stack each time, eventually leading to a stack overflow/exception 1. But, that only happens if it needs to loop, which wasn't the case on dolphin since DMAs complete instantly, so it only crashed like that on real hardware.

[iwubcode]

I'm not familiar with this code, this isn't needed anymore because we early out and don't add the new label?

[Pokechu22]

Yeah, I changed the behavior from replacing the old label with a new one to not replacing the label.

I also think the behavior was buggy if the old label value matched the new one, as then there would be 2 labels in the list with the same value.

DSPTool has an option to continue assembling even if errors occur, so we do need to choose a behavior.

[iwubcode]

Untested but LGTM