lib: tls: add error handling for missing servername
lib: tls: add error handling for missing servername

when writing test-tls-client-reject I was hitting the low level
tls_binding:connect code error handling which isn't very pretty. Print a
nicer message from lua.

GabrielNicolasAvellaneda/luvit-upstream,kaustavha/luvit,boundary/luvit,connectFree/lev,boundary/luvit,sousoux/luvit,GabrielNicolasAvellaneda/luvit-upstream,DBarney/luvit,rjeli/luvit,rjeli/luvit,bsn069/luvit,sousoux/luvit,luvit/luvit,DBarney/luvit,GabrielNicolasAvellaneda/luvit-upstream,AndrewTsao/luvit,rjeli/luvit,kaustavha/luvit,rjeli/luvit,bsn069/luvit,sousoux/luvit,DBarney/luvit,DBarney/luvit,boundary/luvit,bsn069/luvit,AndrewTsao/luvit,AndrewTsao/luvit,kaustavha/luvit,boundary/luvit,sousoux/luvit,zhaozg/luvit,luvit/luvit,sousoux/luvit,GabrielNicolasAvellaneda/luvit-upstream,sousoux/luvit,zhaozg/luvit,connectFree/lev,boundary/luvit,boundary/luvit