Artifact cache: Mention the remote URL when we fail to fetch remote refs

Small change to an error message which slowed me down while trying to set up an artifact cache. When we can't pull refs from the pull_url specified in sshd_config, BuildStream gives an error like this:

Failed to fetch remote refs from ssh://artifacts@selectron.dyn.ducie.codethink.co.uk/artifacts: Could not connect: Connection refused

This indicates that the problem is with the SSH connection, but it's actually with the HTTPS connection discovered via SSH. Hence I've altered the error message to include the pull URL as well.

We don't appear to have any tests for ostree remote caches at the moment, and adding one is a large job so I haven't added any tests here.