Add validation that interactivity layer exists.
Add validation that interactivity layer exists.

Teino1978-Corp/Teino1978-Corp-tilemill,paulovieira/tilemill-clima,paulovieira/tilemill-clima,paulovieira/tilemill-clima,Zhao-Qi/tilemill,mbrukman/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Zhao-Qi/tilemill,mbrukman/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,mbrukman/tilemill,tilemill-project/tilemill,MappingKat/tilemill,tizzybec/tilemill,nyimbi/tilemill,fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,MappingKat/tilemill,nyimbi/tilemill,fxtentacle/tilemill,nyimbi/tilemill,tizzybec/tilemill,tizzybec/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,nyimbi/tilemill,Zhao-Qi/tilemill,florianf/tileoven,fxtentacle/tilemill,tizzybec/tilemill,isaacs/tilemill,tilemill-project/tilemill,tilemill-project/tilemill,MappingKat/tilemill,isaacs/tilemill,paulovieira/tilemill-clima,paulovieira/tilemill-clima,mbrukman/tilemill,Zhao-Qi/tilemill,mbrukman/tilemill,florianf/tileoven,florianf/tileoven,isaacs/tilemill,MappingKat/tilemill,fxtentacle/tilemill,tizzybec/tilemill,MappingKat/tilemill,nyimbi/tilemill