clean up a failed-production block; attempt to catch bad signing key errors at start_block instead of produce_block EOSIO/eos#3161
clean up a failed-production block; attempt to catch bad signing key errors at start_block instead of produce_block EOSIO/eos#3161

EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos