Send deal rejection reason to client

[dirkmc]

Instead of just closing the connection:

$ ./boost deal \
--provider=t01000 \
--commp=baga6ea4seaqhiabj4lnvg5nfgoe6xufk3qwdl54wq7qsu7fqrk3vu6axxukccmq \
--piece-size=2048 \
--car-size=1101 \
--payload-cid=bafk2bzacearocrcaflv5hesscsbiv64x4om5m64nsh347wedig3nn5l3anypi \
--verified=false \
--http-url=http://localhost:5000/1k-file.car
2022-04-12T11:46:35.278+0200	FATAL	boost	boost/main.go:41
	deal proposal rejected: server error: provider has insufficient funds to accept deal

Fixes #433