Fix the CertificateToDane return value: add error
Fix the CertificateToDane return value: add error

This is more inline with the rest of the functions which
do return an actual error. It is however a small api change.

miekg/dns,rolandshoemaker/dns,asergeyev/dns,FiloSottile/dns,ym/go-dns,josephyzhou/dns,jdef/dns,ezoic/dns,Git-Host/dns,letsencrypt/dns,bboreham/dns,flebarbe/dns,AlexCheong/dns,bragr/dns,jmhodges/dns,Psiphon-Inc/dns,hugozhu/dns,tempbottle/dns,tsenart/dns,modulexcite/dns,tonnerre/golang-dns,janstk/dns,benbearchen/dns,cloudflare/dns,curtiszimmerman/dns,ErikDubbelboer/dns,michaelharo/dns,skyportsystems/dns,skynet/dns