improve error messages
improve error messages

peertracksinc/muse,cryptonomex/graphene,bigoc/openchain,bigoc/openchain,cryptonomex/graphene,bigoc/openchain,oxarbitrage/bitshares-core,peertracksinc/muse,cryptonomex/graphene,pmconrad/graphene,pmconrad/graphene,peertracksinc/muse,abitmore/bitshares-2,oxarbitrage/bitshares-core,bitsuperlab/cpp-play2,abitmore/bitshares-2,oxarbitrage/bitshares-core,abitmore/bitshares-2,bitshares/bitshares-2,pmconrad/graphene,bitshares/bitshares-2,bitsuperlab/cpp-play2,bitshares/bitshares-2,pmconrad/graphene,bitsuperlab/cpp-play2,bigoc/openchain,abitmore/bitshares-2,peertracksinc/muse,oxarbitrage/bitshares-core,bitshares/bitshares-2