Shorten the warnings raised when remote cache initialization fails

Sam Thursfieldrequested to merge
sam/improve-cache-warnings into master

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

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