improved error flow in miner.SubmitBlock
improved error flow in miner.SubmitBlock

There was an uneeded `if err == nil {` which both breaks conventions and
made the code more verbose than necessary.

MrStrong/Sia,NebulousLabs/Sia,johnathanhowell/Sia,ChrisSchinnerl/Sia,mtlynch/Sia,mharkus/Sia,ericflo/Sia,Mingling94/Sia,mnsl/Sia,NebulousLabs/Sia,rivine/rivine,MrStrong/Sia,ericflo/Sia,mnsl/Sia,mharkus/Sia,rivine/rivine,ChrisSchinnerl/Sia,mtlynch/Sia