Better json parsing errors

[Ankur Goyal (ankrgyl)]

If a query result fails to parse, we'll get better errors:

error: failed to parse response from POST /btql
  target: bt::sql::SqlResponse
  JSON path: realtime_state.read_bytes
  reason: invalid type: string "oops", expected u64 at line 1 column 72
  status: 200 OK
  content-type: application/json
  body bytes: 74