Audit list of things that can raise ResolveError

[michaelkipper]

#230 surfaced a failure that was not marked by Semian to raise a ResolveError.

https://github.com/Shopify/semian/pull/230#pullrequestreview-250070212 suggested grepping to find more of these messages but there's little confidence that it's 100% complete.

We need to come up with a more systematic way of covering these failures.

[thegedge]

Another thought, related to DNS: Perhaps we should have a DNS adapter that is separate from the others, and uses a separate resource name, since it's a different upstream. This would allow it to be configured differently from the other adapters, which could make it easier to recover from.