Catch exception properly when DNS answers with "empty" response.

[uwekamper]

In some cases, the unbound in the docker setup responds with an empty list and dnspython throws an exception:

smtp_dane_verify.dns_records.MxRecordError: An error occurred: All nameservers failed to answer the query some-domain.com. IN MX: Server Do53:172.18.0.3@53 answered SERVFAIL

smtp-dane-verify should report the error properly. Currently it only responds with "Internal server error".