Improve the `emptyDNSResponseError` message. (#2812)
Improve the `emptyDNSResponseError` message. (#2812)

When the RA's `validateEmail` function performs DNS lookups for the MX
and A records for the email domain it returns the
`emptyDNSResponseError` message if neither an MX or A record are found.

Prior to this commit the message only said "empty DNS response" and
didn't indicate what the DNS lookup was being used for, or which records
were empty. This commit updates the message to hopefully be less
confusing and more immediately actionable by a user.
letsencrypt/boulder,jfrazelle/boulder,bmw/boulder,bmw/boulder,letsencrypt/boulder,jfrazelle/boulder,jfrazelle/boulder,letsencrypt/boulder,jsha/boulder,jcjones/boulder,ibukanov/boulder,lucasallan/boulder,bmw/boulder,jcjones/boulder,jsha/boulder,ibukanov/boulder,jcjones/boulder,ibukanov/boulder,jsha/boulder,lucasallan/boulder,lucasallan/boulder,jcjones/boulder,jfrazelle/boulder