BUG without stack trace when public keys are not specified for an artifact cache using a self-signed certificate

Summary

Steps to reproduce

This issue can be reproduced by configuring an artifact cache without providing the server-cert and client-cert fields.

artifacts:
  url: https://cache.trustable.io:11002
  client-key: certificates/client.key
  push: true

What is the current bug behavior?

A bug is printed without any stack trace. The bug is not particularly meaningful.

[--:--:--][][] BUG     expected bytes, NoneType found

    TypeError: expected bytes, NoneType found

See full log below.

What is the expected correct behavior?

It is expected that a readable error would be produced. According to #buildstream it's also expected that a stack trace would be produced.

Relevant logs and/or screenshots

https://gitlab.com/trustable/distros/minimal-distro/-/jobs/94525056

Possible fixes

This isn't really a possible fix, but it'd be good if the documentation could be updated to emphasize the importance of providing public keys for self-signed certificates.

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.1.7

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