[RFE] Add option to make build fail if the configured cache server is down

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/219)
In GitLab by [[Gitlab user @jjardon]](https://gitlab.com/jjardon) on Jan 29, 2018, 15:02

Currently it only generates a warning but the build continues

This is bad for us because builds that could take a few minutes can span for hours (We can not limit the builds to minutes because it perfectly valid in some situations that a build takes several hours)

Start by reading the linked GitLab issue and locating BuildStream's cache-server failure handling, especially where the current warning is emitted. Add an option that makes the build fail when the configured cache server is down, while retaining the current warning-and-continue behavior by default; verify both paths with the relevant tests once identified.

Written by the indexing model from the issue text.