[scripts] previous_release: improve failed download error message
[scripts] previous_release: improve failed download error message

before:
------------------------------------------------------------
$ contrib/devtools/previous_release.sh -r -b v0.9.5
[...]
gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
------------------------------------------------------------

now:
------------------------------------------------------------
$ contrib/devtools/previous_release.sh -r -b v0.9.5
[...]
curl: (22) The requested URL returned error: 404 Not Found
Download failed.
------------------------------------------------------------

domob1812/bitcoin,domob1812/bitcoin,midnightmagic/bitcoin,qtumproject/qtum,jambolo/bitcoin,litecoin-project/litecoin,mruddy/bitcoin,particl/particl-core,qtumproject/qtum,practicalswift/bitcoin,ahmedbodi/vertcoin,apoelstra/bitcoin,alecalve/bitcoin,instagibbs/bitcoin,JeremyRubin/bitcoin,rnicoll/bitcoin,peercoin/peercoin,namecoin/namecore,prusnak/bitcoin,midnightmagic/bitcoin,jambolo/bitcoin,bitcoin/bitcoin,alecalve/bitcoin,achow101/bitcoin,ahmedbodi/vertcoin,mruddy/bitcoin,sstone/bitcoin,vertcoin/vertcoin,prusnak/bitcoin,bitcoinsSG/bitcoin,lateminer/bitcoin,pataquets/namecoin-core,ahmedbodi/vertcoin,rnicoll/dogecoin,JeremyRubin/bitcoin,MeshCollider/bitcoin,vertcoin/vertcoin,n1bor/bitcoin,prusnak/bitcoin,MeshCollider/bitcoin,n1bor/bitcoin,kallewoof/bitcoin,bitcoin/bitcoin,domob1812/namecore,pstratem/bitcoin,GroestlCoin/bitcoin,peercoin/peercoin,litecoin-project/litecoin,Xekyo/bitcoin,instagibbs/bitcoin,pataquets/namecoin-core,bitcoinsSG/bitcoin,pstratem/bitcoin,bitcoinknots/bitcoin,Xekyo/bitcoin,midnightmagic/bitcoin,litecoin-project/litecoin,jamesob/bitcoin,fujicoin/fujicoin,sipsorcery/bitcoin,namecoin/namecoin-core,fujicoin/fujicoin,namecoin/namecoin-core,jamesob/bitcoin,domob1812/bitcoin,jonasschnelli/bitcoin,midnightmagic/bitcoin,lateminer/bitcoin,practicalswift/bitcoin,ajtowns/bitcoin,alecalve/bitcoin,lateminer/bitcoin,EthanHeilman/bitcoin,MeshCollider/bitcoin,mruddy/bitcoin,domob1812/namecore,vertcoin/vertcoin,namecoin/namecore,practicalswift/bitcoin,GroestlCoin/bitcoin,fujicoin/fujicoin,sipsorcery/bitcoin,GroestlCoin/GroestlCoin,GroestlCoin/bitcoin,monacoinproject/monacoin,anditto/bitcoin,sstone/bitcoin,qtumproject/qtum,jonasschnelli/bitcoin,namecoin/namecore,ajtowns/bitcoin,peercoin/peercoin,tecnovert/particl-core,GroestlCoin/GroestlCoin,dscotese/bitcoin,rnicoll/dogecoin,domob1812/namecore,DigitalPandacoin/pandacoin,sstone/bitcoin,monacoinproject/monacoin,jonasschnelli/bitcoin,ElementsProject/elements,ajtowns/bitcoin,jnewbery/bitcoin,lateminer/bitcoin,JeremyRubin/bitcoin,sipsorcery/bitcoin,jambolo/bitcoin,MarcoFalke/bitcoin,particl/particl-core,ElementsProject/elements,sipsorcery/bitcoin,DigitalPandacoin/pandacoin,practicalswift/bitcoin,prusnak/bitcoin,particl/particl-core,AkioNak/bitcoin,practicalswift/bitcoin,AkioNak/bitcoin,anditto/bitcoin,lateminer/bitcoin,bitcoinsSG/bitcoin,rnicoll/bitcoin,instagibbs/bitcoin,AkioNak/bitcoin,apoelstra/bitcoin,peercoin/peercoin,MeshCollider/bitcoin,mm-s/bitcoin,ahmedbodi/vertcoin,particl/particl-core,jlopp/statoshi,achow101/bitcoin,midnightmagic/bitcoin,AkioNak/bitcoin,rnicoll/bitcoin,tecnovert/particl-core,bitcoin/bitcoin,andreaskern/bitcoin,anditto/bitcoin,DigitalPandacoin/pandacoin,practicalswift/bitcoin,JeremyRubin/bitcoin,pataquets/namecoin-core,jambolo/bitcoin,mm-s/bitcoin,EthanHeilman/bitcoin,DigitalPandacoin/pandacoin,monacoinproject/monacoin,n1bor/bitcoin,rnicoll/bitcoin,kallewoof/bitcoin,GroestlCoin/GroestlCoin,domob1812/namecore,andreaskern/bitcoin,yenliangl/bitcoin,Sjors/bitcoin,yenliangl/bitcoin,AkioNak/bitcoin,JeremyRubin/bitcoin,pstratem/bitcoin,pataquets/namecoin-core,instagibbs/bitcoin,domob1812/bitcoin,andreaskern/bitcoin,instagibbs/bitcoin,fanquake/bitcoin,dscotese/bitcoin,qtumproject/qtum,achow101/bitcoin,EthanHeilman/bitcoin,GroestlCoin/bitcoin,pataquets/namecoin-core,cdecker/bitcoin,mruddy/bitcoin,cdecker/bitcoin,sstone/bitcoin,apoelstra/bitcoin,EthanHeilman/bitcoin,Xekyo/bitcoin,jnewbery/bitcoin,tecnovert/particl-core,namecoin/namecore,ahmedbodi/vertcoin,MeshCollider/bitcoin,jamesob/bitcoin,ElementsProject/elements,fujicoin/fujicoin,Xekyo/bitcoin,sstone/bitcoin,ajtowns/bitcoin,particl/particl-core,jlopp/statoshi,MarcoFalke/bitcoin,peercoin/peercoin,apoelstra/bitcoin,jambolo/bitcoin,monacoinproject/monacoin,prusnak/bitcoin,jlopp/statoshi,litecoin-project/litecoin,bitcoin/bitcoin,GroestlCoin/bitcoin,anditto/bitcoin,tecnovert/particl-core,domob1812/bitcoin,mruddy/bitcoin,jnewbery/bitcoin,pataquets/namecoin-core,Sjors/bitcoin,bitcoinknots/bitcoin,Sjors/bitcoin,ajtowns/bitcoin,litecoin-project/litecoin,peercoin/peercoin,n1bor/bitcoin,qtumproject/qtum,achow101/bitcoin,bitcoin/bitcoin,sipsorcery/bitcoin,domob1812/namecore,instagibbs/bitcoin,qtumproject/qtum,andreaskern/bitcoin,yenliangl/bitcoin,rnicoll/bitcoin,ajtowns/bitcoin,jamesob/bitcoin,GroestlCoin/GroestlCoin,jamesob/bitcoin,MarcoFalke/bitcoin,domob1812/bitcoin,GroestlCoin/GroestlCoin,domob1812/namecore,Sjors/bitcoin,kallewoof/bitcoin,kallewoof/bitcoin,MarcoFalke/bitcoin,EthanHeilman/bitcoin,jonasschnelli/bitcoin,namecoin/namecoin-core,alecalve/bitcoin,achow101/bitcoin,cdecker/bitcoin,Xekyo/bitcoin,dscotese/bitcoin,apoelstra/bitcoin,lateminer/bitcoin,sstone/bitcoin,anditto/bitcoin,fanquake/bitcoin,andreaskern/bitcoin,n1bor/bitcoin,dscotese/bitcoin,bitcoinsSG/bitcoin,cdecker/bitcoin,dscotese/bitcoin,namecoin/namecore,yenliangl/bitcoin,pstratem/bitcoin,MarcoFalke/bitcoin,rnicoll/dogecoin,cdecker/bitcoin,vertcoin/vertcoin,bitcoin/bitcoin,JeremyRubin/bitcoin,DigitalPandacoin/pandacoin,bitcoinknots/bitcoin,anditto/bitcoin,AkioNak/bitcoin,fanquake/bitcoin,mruddy/bitcoin,kallewoof/bitcoin,mm-s/bitcoin,namecoin/namecoin-core,rnicoll/dogecoin,GroestlCoin/bitcoin,apoelstra/bitcoin,ahmedbodi/vertcoin,jlopp/statoshi,andreaskern/bitcoin,rnicoll/bitcoin,midnightmagic/bitcoin,pstratem/bitcoin,namecoin/namecore,bitcoinknots/bitcoin,rnicoll/dogecoin,ElementsProject/elements,litecoin-project/litecoin,vertcoin/vertcoin,sipsorcery/bitcoin,jonasschnelli/bitcoin,ElementsProject/elements,n1bor/bitcoin,bitcoinknots/bitcoin,namecoin/namecoin-core,fanquake/bitcoin,Sjors/bitcoin,mm-s/bitcoin,jambolo/bitcoin,kallewoof/bitcoin,mm-s/bitcoin,alecalve/bitcoin,monacoinproject/monacoin,qtumproject/qtum,bitcoinsSG/bitcoin,particl/particl-core,jnewbery/bitcoin,jamesob/bitcoin,mm-s/bitcoin,yenliangl/bitcoin,MeshCollider/bitcoin,tecnovert/particl-core,jnewbery/bitcoin,MarcoFalke/bitcoin,alecalve/bitcoin,EthanHeilman/bitcoin,Xekyo/bitcoin,achow101/bitcoin,tecnovert/particl-core,fanquake/bitcoin,fujicoin/fujicoin,fujicoin/fujicoin,ElementsProject/elements,namecoin/namecoin-core,bitcoinsSG/bitcoin,yenliangl/bitcoin,fanquake/bitcoin,DigitalPandacoin/pandacoin,cdecker/bitcoin,pstratem/bitcoin,prusnak/bitcoin,GroestlCoin/GroestlCoin,jlopp/statoshi,jlopp/statoshi,vertcoin/vertcoin,monacoinproject/monacoin,dscotese/bitcoin