test: check parameter validity in rpc_signmessage.py
test: check parameter validity in rpc_signmessage.py

checks parameter validity and error codes for the related RPCs:
    - signmessagewithprivkey
    - signmessage
    - verifymessage

GroestlCoin/GroestlCoin,namecoin/namecore,qtumproject/qtum,GroestlCoin/GroestlCoin,jamesob/bitcoin,namecoin/namecoin-core,MeshCollider/bitcoin,qtumproject/qtum,domob1812/bitcoin,mm-s/bitcoin,MarcoFalke/bitcoin,mruddy/bitcoin,sipsorcery/bitcoin,GroestlCoin/bitcoin,tecnovert/particl-core,fujicoin/fujicoin,rnicoll/bitcoin,dscotese/bitcoin,jamesob/bitcoin,MarcoFalke/bitcoin,bitcoin/bitcoin,rnicoll/bitcoin,jamesob/bitcoin,namecoin/namecore,qtumproject/qtum,kallewoof/bitcoin,namecoin/namecoin-core,rnicoll/bitcoin,mruddy/bitcoin,dscotese/bitcoin,kallewoof/bitcoin,fujicoin/fujicoin,Xekyo/bitcoin,tecnovert/particl-core,anditto/bitcoin,dscotese/bitcoin,MarcoFalke/bitcoin,jlopp/statoshi,pataquets/namecoin-core,sstone/bitcoin,lateminer/bitcoin,instagibbs/bitcoin,achow101/bitcoin,apoelstra/bitcoin,andreaskern/bitcoin,JeremyRubin/bitcoin,n1bor/bitcoin,anditto/bitcoin,domob1812/namecore,prusnak/bitcoin,lateminer/bitcoin,bitcoinknots/bitcoin,namecoin/namecoin-core,instagibbs/bitcoin,GroestlCoin/GroestlCoin,jambolo/bitcoin,apoelstra/bitcoin,apoelstra/bitcoin,GroestlCoin/bitcoin,pataquets/namecoin-core,andreaskern/bitcoin,mm-s/bitcoin,AkioNak/bitcoin,ElementsProject/elements,ajtowns/bitcoin,GroestlCoin/bitcoin,ajtowns/bitcoin,andreaskern/bitcoin,ajtowns/bitcoin,apoelstra/bitcoin,particl/particl-core,prusnak/bitcoin,qtumproject/qtum,kallewoof/bitcoin,qtumproject/qtum,pataquets/namecoin-core,fujicoin/fujicoin,MeshCollider/bitcoin,Sjors/bitcoin,kallewoof/bitcoin,dscotese/bitcoin,kallewoof/bitcoin,achow101/bitcoin,particl/particl-core,andreaskern/bitcoin,sstone/bitcoin,bitcoinknots/bitcoin,sipsorcery/bitcoin,n1bor/bitcoin,bitcoinknots/bitcoin,MarcoFalke/bitcoin,instagibbs/bitcoin,particl/particl-core,domob1812/namecore,Xekyo/bitcoin,jlopp/statoshi,bitcoinsSG/bitcoin,bitcoinsSG/bitcoin,mruddy/bitcoin,prusnak/bitcoin,pataquets/namecoin-core,instagibbs/bitcoin,prusnak/bitcoin,ElementsProject/elements,sstone/bitcoin,bitcoinknots/bitcoin,jlopp/statoshi,n1bor/bitcoin,yenliangl/bitcoin,bitcoin/bitcoin,yenliangl/bitcoin,Sjors/bitcoin,n1bor/bitcoin,bitcoinsSG/bitcoin,ajtowns/bitcoin,mm-s/bitcoin,AkioNak/bitcoin,jamesob/bitcoin,lateminer/bitcoin,jambolo/bitcoin,fanquake/bitcoin,sstone/bitcoin,qtumproject/qtum,sstone/bitcoin,anditto/bitcoin,domob1812/bitcoin,achow101/bitcoin,sipsorcery/bitcoin,lateminer/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,domob1812/namecore,GroestlCoin/GroestlCoin,n1bor/bitcoin,sstone/bitcoin,mruddy/bitcoin,apoelstra/bitcoin,ElementsProject/elements,bitcoinknots/bitcoin,sipsorcery/bitcoin,bitcoin/bitcoin,domob1812/bitcoin,ElementsProject/elements,namecoin/namecore,JeremyRubin/bitcoin,yenliangl/bitcoin,yenliangl/bitcoin,rnicoll/bitcoin,dscotese/bitcoin,bitcoinsSG/bitcoin,AkioNak/bitcoin,AkioNak/bitcoin,namecoin/namecoin-core,particl/particl-core,mm-s/bitcoin,sipsorcery/bitcoin,MeshCollider/bitcoin,domob1812/bitcoin,GroestlCoin/bitcoin,anditto/bitcoin,bitcoinsSG/bitcoin,sipsorcery/bitcoin,fujicoin/fujicoin,fanquake/bitcoin,pataquets/namecoin-core,instagibbs/bitcoin,domob1812/namecore,ajtowns/bitcoin,jlopp/statoshi,jamesob/bitcoin,jambolo/bitcoin,JeremyRubin/bitcoin,achow101/bitcoin,prusnak/bitcoin,Sjors/bitcoin,andreaskern/bitcoin,domob1812/bitcoin,MeshCollider/bitcoin,yenliangl/bitcoin,MeshCollider/bitcoin,jamesob/bitcoin,rnicoll/bitcoin,n1bor/bitcoin,namecoin/namecoin-core,particl/particl-core,fujicoin/fujicoin,GroestlCoin/GroestlCoin,GroestlCoin/GroestlCoin,MeshCollider/bitcoin,bitcoinsSG/bitcoin,GroestlCoin/bitcoin,yenliangl/bitcoin,tecnovert/particl-core,Sjors/bitcoin,namecoin/namecore,tecnovert/particl-core,Xekyo/bitcoin,anditto/bitcoin,ElementsProject/elements,mm-s/bitcoin,bitcoin/bitcoin,achow101/bitcoin,jambolo/bitcoin,jlopp/statoshi,jambolo/bitcoin,fanquake/bitcoin,prusnak/bitcoin,anditto/bitcoin,tecnovert/particl-core,JeremyRubin/bitcoin,MarcoFalke/bitcoin,Xekyo/bitcoin,Sjors/bitcoin,GroestlCoin/bitcoin,namecoin/namecore,ElementsProject/elements,apoelstra/bitcoin,instagibbs/bitcoin,fanquake/bitcoin,AkioNak/bitcoin,lateminer/bitcoin,mruddy/bitcoin,qtumproject/qtum,particl/particl-core,namecoin/namecore,mruddy/bitcoin,pataquets/namecoin-core,Xekyo/bitcoin,bitcoin/bitcoin,domob1812/bitcoin,bitcoin/bitcoin,JeremyRubin/bitcoin,fujicoin/fujicoin,MarcoFalke/bitcoin,namecoin/namecoin-core,dscotese/bitcoin,fanquake/bitcoin,rnicoll/bitcoin,JeremyRubin/bitcoin,jlopp/statoshi,mm-s/bitcoin,achow101/bitcoin,ajtowns/bitcoin,AkioNak/bitcoin,tecnovert/particl-core,domob1812/namecore,jambolo/bitcoin,lateminer/bitcoin,andreaskern/bitcoin,domob1812/namecore,Xekyo/bitcoin