[RPC] validate proposal amount earlier
[RPC] validate proposal amount earlier

the requested proposal payout amount can be checked earlier in the
command pipeline; returns a more detailed message on error.

PIVX-Project/PIVX,Mrs-X/Darknet,Mrs-X/PIVX,Mrs-X/PIVX,Darknet-Crypto/Darknet,Mrs-X/Darknet,Darknet-Crypto/Darknet,Mrs-X/Darknet,Mrs-X/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Mrs-X/PIVX,PIVX-Project/PIVX,Mrs-X/Darknet,Mrs-X/PIVX,PIVX-Project/PIVX,Mrs-X/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Mrs-X/PIVX,Darknet-Crypto/Darknet,Mrs-X/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet