core: add detail for invalid request body error msg
core: add detail for invalid request body error msg

The ErrBadRequest error is a bit vague. Although we cannot
give a more granular answer, this commit gives the client's
developer a hint. Mentioned here: https://trello.com/c/lD4Lr4vf/

Closes chain/chainprv#944.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@erykwalder

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain