Error when pushing to the CAS server: "Name resolution failure"

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/710)
In GitLab by [[Gitlab user @jjardon]](https://gitlab.com/jjardon) on Oct 11, 2018, 22:25

## Summary
I got the following backtrace when pushing to the CAS server: https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/107204101

## Steps to reproduce
- I have only see this one once; I will update this if I see it again

## What is the expected correct behavior?
- Handle this gracefully

* BuildStream version affected: buildstream 1.2.3 on both server and client

Start with the linked job backtrace and trace the CAS push path in BuildStream 1.2.3 on both the client and server. The failure is not currently reproducible, so first determine where the name-resolution error is raised and how it can be handled gracefully; done means the push no longer fails with an unhandled error.

Written by the indexing model from the issue text.