Improve logging of failures in the VA
Improve logging of failures in the VA

There's a consistent format to the messages to aid in searching. They
start with [va-err-log], and if an error value is present, end with
Err=%#v.

jsha/boulder,ZCloud-Firstserver/boulder,letsencrypt/boulder,mehmooda/boulder,lucasallan/boulder,benileo/boulder,patf/boulder,jsha/boulder,sophie-h/boulder,benileo/boulder,jsha/boulder,ibukanov/boulder,jcjones/boulder,lucasallan/boulder,sophie-h/boulder,jfrazelle/boulder,jcjones/boulder,patf/boulder,jfrazelle/boulder,letsencrypt/boulder,ZCloud-Firstserver/boulder,ibukanov/boulder,bmw/boulder,lucasallan/boulder,mehmooda/boulder,sophie-h/boulder,patf/boulder,bmw/boulder,jcjones/boulder,mehmooda/boulder,benileo/boulder,benileo/boulder,bmw/boulder,letsencrypt/boulder,mehmooda/boulder,jcjones/boulder,ibukanov/boulder,ZCloud-Firstserver/boulder,jfrazelle/boulder,patf/boulder,ZCloud-Firstserver/boulder,jfrazelle/boulder