Investigate intermittent errors when adding domains that support MTA-STS.

Initially reported at EFForg/starttls-everywhere#146:

> When I'm trying to submit my domain with an MTA STS policy at https://starttls-everywhere.org/add-domain/ I get the following error:
>
> "Domain does not correctly implement MTA-STS."
>
> The error comes from an API endpoint at https://starttls-everywhere.org/api/queue
>
> I am not aware that anything is wrong with my MTA-STS setup. I also would strongly suggest to provide more meaningful error messages.

Start at the /api/queue endpoint described in the report and reproduce submissions for domains with MTA-STS policies. Trace how the checker produces “Domain does not correctly implement MTA-STS” and compare intermittent failures with successful requests. Done means the failure cause is identified and the resulting error behavior is meaningfully explained or corrected.

Written by the indexing model from the issue text.