peer: Don't log write errs due to disconnect.
peer: Don't log write errs due to disconnect.

This commit modifies the peer logging so that it will not log an error
when it's due to the remote peer disconnecting or the peer being
forcibly disconnected or shutdown.

jrick/btcd,jrick/btcd,jcmincke/dcrd,runeaune/btcd,qchain/btcd,wallclockbuilder/btcd,aakselrod/btcd,btcsuite/btcd,ppcsuite/ppcd,Roasbeef/btcd,decred/dcrd,wallclockbuilder/btcd,adiabat/btcd,dan-da/btcd,tb00/btcd,decred/dcrd,runeaune/btcd,cqswxj2013/btcd,Roasbeef/btcd,stevenroose/btcd,jrick/dcrd,qchain/btcd,stevenroose/btcd,dajohi/btcd,jongillham/btcd,dajohi/btcd,btcsuite/btcd,ltcsuite/ltcd,jrick/dcrd,soapboxsys/ombfullnode,hectorj/btcd,ltcsuite/ltcd,ppcsuite/ppcd,cqswxj2013/btcd,hectorj/btcd,tb00/btcd,adiabat/btcd,Stash-Crypto/btcd,jrick/btcd,jongillham/btcd,davecgh/btcd,aakselrod/btcd,BitGo/prova,jcmincke/dcrd,dajohi/btcd,jcmincke/dcrd,davecgh/btcd,soapboxsys/ombfullnode,BitGo/prova,Stash-Crypto/btcd,dan-da/btcd