qbt fetchPins fails with unhelpful error when ssh is not set up

[terabyte]

Many of our example configs use a remote like git@github.com:TerabyteQbt/%r or a GithubQbtRemote that points to TerabyteQbt. In either case, this will fail if the ssh server fingerprint is not already accepted, if the ssh key used is not already known by github, etc etc.

1. GithubQbtRemote should probably use https for all reads, not ssh.
2. When ssh is not set up correctly, it'd be nice to detect that problem and report it to the user. Since fetchPins often tries to fetch for every repo and some may be missing from a given remote (i.e. I have a repo that amling does not) it may take some thought about how to distinguish between these errors. Perhaps do a test fetch first, unclear.