Make JSON parsing error messages more uniform
Make JSON parsing error messages more uniform

pjulien/capnproto,rcrowder/capnproto,mologie/capnproto,hntrmrrs/capnproto,jparyani/capnproto,mrdomino/capnproto,maurer/capnproto,maurer/capnproto,vladon/capnproto,jparyani/capnproto,pjulien/capnproto,rcrowder/capnproto,pjulien/capnproto,hntrmrrs/capnproto,maurer/capnproto,vladon/capnproto,mologie/capnproto,hntrmrrs/capnproto,mrdomino/capnproto,mrdomino/capnproto,mologie/capnproto,vladon/capnproto,rcrowder/capnproto,jparyani/capnproto