Fix false reporting of (plain) vsn strings
Fix false reporting of (plain) vsn strings

rebar used to mistakenly report plain version strings like
{vsn, "1.0.0"} as follows:
DEBUG: vcs_vsn: Unknown VCS atom in vsn field: "1.0.0"

Properly detect unknown/unsupported version terms and abort
if we encounter one.

While at it, rename a variable in vcs_vsn/3 to be non-misleading.

lpgauth/rebar,benoitc/rebar3,K2InformaticsGmbH/rebar,surik/rebar3,project-fifo/rebar3,lrascao/rebar,matwey/rebar,fishcakez/rebar3,saleyn/rebar3,bsmr-erlang/rebar,linearregression/rebar3,jbalint/rebar3,platinumthinker/rebar,project-fifo/rebar3,oubiwann/rebar,norton/rebar3,ferd/rebar3,GabrielNicolasAvellaneda/rebar,lrascao/rebar,cloudant/couchdb-rebar,cloudant/couchdb-rebar,beni55/rebar,lrascao/rebar3,lpgauth/rebar,paulo-ferraz-oliveira/rebar,GabrielNicolasAvellaneda/rebar,rebar/rebar,fauxsoup/rebar3,lrascao/rebar,scatterbrain/rebar,apache/couchdb-rebar,CMoH/rebar,macintux/rebar,jacktang/rebar,rebar/rebar,rebar/rebar,surik/rebar3,zugolosian/rebar3,evnix/rebar,matwey/rebar,blt/rebar3,K2InformaticsGmbH/rebar,jacktang/rebar,paulo-ferraz-oliveira/rebar,cloudant/couchdb-rebar,dcarastan/rebar,syl20bnr/rebar,beni55/rebar,vladdu/rebar3,jacktang/rebar,oubiwann/rebar,zugolosian/rebar3,beni55/rebar,K2InformaticsGmbH/rebar,kellymclaughlin/rebar3,processone/rebar3,RubberCthulhu/rebar,CMoH/rebar,K2InformaticsGmbH/rebar,linearregression/rebar3,walrusVision/rebar3,bsmr-erlang/rebar,heroku/rebar,platinumthinker/rebar,shkumagai/rebar,evnix/rebar,rebar/rebar3,sile/rebar3,lpgauth/rebar,scatterbrain/rebar,benoitc/rebar3,heroku/rebar,apache/couchdb-rebar,erlang/rebar3,paulo-ferraz-oliveira/rebar,processone/rebar3,choptastic/rebar,matwey/rebar,bsmr-erlang/rebar,sile/rebar3,walrusVision/rebar3,fabiankrol/rebar3,apache/couchdb-rebar,SiDen/rebar,macintux/rebar,dcarastan/rebar,K2InformaticsGmbH/rebar,blt/rebar3,RubberCthulhu/rebar,oubiwann/rebar,CMoH/rebar,fauxsoup/rebar3,syl20bnr/rebar,fabiankrol/rebar3,norton/rebar,norton/rebar,choptastic/rebar,matwey/rebar,djnym/rebar3,jbalint/rebar3,SiDen/rebar,rebar/rebar3,csyangchen/rebar,evnix/rebar,syl20bnr/rebar,choptastic/rebar,norton/rebar,lrascao/rebar,macintux/rebar,bsmr-erlang/rebar,scatterbrain/rebar,matwey/rebar,lrascao/rebar,SiDen/rebar,norton/rebar,csyangchen/rebar,csyangchen/rebar,GabrielNicolasAvellaneda/rebar,shkumagai/rebar,shkumagai/rebar,tsloughter/rebar3,platinumthinker/rebar,dcarastan/rebar