Unhandled git failures during clone/pull

Currently running into a weird error where columbia's repository won't update:
```
Git::FailedError: [...], status: pid 46072 exit 128,
stderr: "error: cannot pull with rebase: You have unstaged changes.
error: Please commit or stash them." (Git::FailedError)
```

At a minumum, we could gracefully handle `Git::FailedError` and keep moving.