Shorten the warnings raised when remote cache initialization fails

Before:

    [--:--:--] WARNING Failed to fetch remote refs from ssh://ostree@ostree.baserock.org:22000/cache: BuildStream did not connect successfully to the shared cache ssh://ostree@ostree.baserock.org:22000/cache: SSH error: ssh: connect to host ostree.baserock.org port 22000: Connection refused

After:

    [--:--:--] WARNING Failed to fetch remote refs from ssh://ostree@ostree.baserock.org:22000/cache. ssh: connect to host ostree.baserock.org port 22000: Connection refused