discovery: check proof for nilness before creating chanAnn
discovery: check proof for nilness before creating chanAnn

This commit makes sure we are not attempting to create a
channel announcement with a nil ChannelAuthProof, as that
could cause a crash at startup whe the gossiper would
attempt to reprocess an edge coming from the fundingmanager.

It also makes sure we check the correct error returned from
processRejectedEdge.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd