Convert transaction pool errors to their own type.
Convert transaction pool errors to their own type.

This allows the caller to distinguish if the error is because of a rule
violation or due to something actually going wrong.

ltcsuite/ltcd,cjepson/btcd,danoctavian/btcd,Evil-Coin/Evil,DanielKrawisz/btcd,Dirbaio/btcd,ltcsuite/ltcd,mathiasrw/btcd,sysbot/btcd,tb00/btcd,soapboxsys/ombfullnode,cqswxj2013/btcd,ppcsuite/ppcd-old,btcsuitereleases/btcd,FactomProject/btcd,cqswxj2013/btcd,Roasbeef/btcd,Dirbaio/btcd,pkothbauer/btcd,pkothbauer/btcd,decred/dcrd,jrick/btcd,ppcsuite/ppcd,adiabat/btcd,btcsuite/btcd,Evil-Coin/Evil,dajohi/btcd,1d4Nf6/btcd,BitGo/prova,stevenroose/btcd,danoctavian/btcd,1d4Nf6/btcd,Stash-Crypto/btcd,wallclockbuilder/btcd,Dirbaio/btcd,FactomProject/btcd,hectorj/btcd,btcsuite/btcd,decred/dcrd,dajohi/btcd,Roasbeef/btcd,soapboxsys/ombfullnode,btcsuitereleases/btcd,Stash-Crypto/btcd,wallclockbuilder/btcd,BitGo/prova,DanielKrawisz/btcd,jcmincke/dcrd,glv2/ppcd,adiabat/btcd,jrick/dcrd,toddfries/btcd,jongillham/btcd,glv2/ppcd,jimmysong/btcd,jongillham/btcd,Cleawing/btcd,Cleawing/btcd,davecgh/btcd,jrick/btcd,jcmincke/dcrd,jimmysong/btcd,ppcsuite/ppcd,dan-da/btcd,aakselrod/btcd,CryptocurrencyCabal/htcd,CryptocurrencyCabal/htcd,aakselrod/btcd,dajohi/btcd,jrick/dcrd,mathiasrw/btcd,jcmincke/dcrd,qchain/btcd,ppcsuite/ppcd-old,davecgh/btcd,runeaune/btcd,hectorj/btcd,runeaune/btcd,cjepson/btcd,jrick/btcd,sysbot/btcd,toddfries/btcd,qchain/btcd,dan-da/btcd,stevenroose/btcd,tb00/btcd