Fixes late boost error message.
Closes front#2722 (closed)
Alternative to this would be to move some validation logic to the boost prepare endpoint, as this code is called AFTER metamask has dispatched the transaction - meaning an error debits the user, but the boost fails until an admin notices and refunds.
The downside to that and why I prefer this approach is to keep the boost prepare endpoint quick, and not lock it up waiting for ES to return a duplicate boost.
Edited by Ben