Add basic validation for exportjob model.
Add basic validation for exportjob model.

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