Adjust osx build shell script to bail if node is missing.
Adjust osx build shell script to bail if node is missing.

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