better error message for derivePersistFieldJSON
better error message for derivePersistFieldJSON

junjihashimoto/persistent,gbwey/persistent,jcristovao/persistent,gbwey/persistent,superduper/persistent,superduper/persistent,plow-technologies/persistent,greydot/persistent,jcristovao/persistent,jasonzoladz/persistent,nakaji-dayo/persistent,andrewthad/persistent,mkscrg/persistent,yesodweb/persistent,nakaji-dayo/persistent,pseudonom/persistent,nakaji-dayo/persistent,jasonzoladz/persistent,bitemyapp/persistent,mkscrg/persistent,rdnetto/persistent,superduper/persistent,creichert/persistent,junjihashimoto/persistent,mitchellwrosen/persistent,mitchellwrosen/persistent,mkscrg/persistent,mitchellwrosen/persistent,jasonzoladz/persistent,meteficha/persistent,erikd/persistent,psibi/persistent,greydot/persistent,naushadh/persistent,meteficha/persistent,meteficha/persistent,andrewthad/persistent,pseudonom/persistent,gbwey/persistent,andrewthad/persistent,rdnetto/persistent,greydot/persistent,paul-rouse/persistent,jcristovao/persistent