Make hex decoding consistent in RPC handlers.
Make hex decoding consistent in RPC handlers.

The hex package requires an even number of characters in hex encoded
strings.  Some of the handlers already prepended a zero if necessary to
make this condition true, however other did not.  This commit extends this
functionality to all handlers and also makes the error return consistent.

ok @jrick

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