[LiskHQ/lisk] Improve param validation logic to merge 400 and 409 status code responses (#1299)

There is a benefit in the current approach. Someone aware of the distinction between 400 and 409 codes meanings could reuse that information:
> 400 — Any error in request validation (like param type, format etc)
> 409 — Any validation error occurred during processing of request in Lisk Core