[BuildStream] unavailable artifact cache fatal warnings

[Benjamin Schubert <ben c schubert gmail com>]

Hey everyone!

When using a remote artifact cache, we issue a warning if we can't reach it. I would like to be able to configure it as an error.

This is already feasible with multiple other warnings. However this warning is more complex since we can define artifact caches in both projects and globally, but warnings are only per project.

 I would see three ways of implementing this:

1) Specify the warning per project, and the toplevel project's configuration is used for global caches

2) Specify the warning per project, and don't allow setting this for global caches

3) Add the warning key in the user's configuration too, with it being used for global caches. Also use this warning configuration for every project that does not specify the warnings-fatal key

What do you think? What would you prefer to see? I like the third option the most, but it is also the most complex to implement. I will create an issue about that once we have consensus.

Cheers,

Benjamin