Handle if there's no remote for the upstream repo

[ecraig12345]

If the user hasn't configured a git remote for the main/upstream repo, beachball should detect this and show an error instructing how to add a remote. Something like:

Could not find a git remote for this package's [main/upstream/???] repo. Run the
following command to add the remote, then try generating change files again:

    git remote add upstream <url here>

(I'm not sure what the standard term is for an original repo when you're working from a fork, and of course the other wording is negotiable too.)

Inspired by discussion starting at microsoft/fluentui#10294 (comment) trying to debug why too many change files were generated for a PR.