Hey,
I never added emoji support to STM, but you can use quads to render emoji. Basically, to support it I'd have to make it so emoji are swapped out for quads before rendering anyway, since the dynamic texture code I'm using doesn't support Unicode. (That's the error you're getting, I believe) but there's just so many emoji, no 100% free emoji sheets I could bundle with STM as an example, and they constantly keep adding new ones, so I never added it as a feature.
Do please check out quads, though! You can get a lot more style and personality out of them, in my opinion! Plus add whatever you want, even if it's not emoji!