Better failure when syncing with uncommitted changes

[alicederyn]

If you run git sync with uncommitted changes on a branch with remote changes (e.g. on main with tool-created changes to .gitignore, in my case), you get an error:

error: cannot pull with rebase: You have unstaged changes.
error: please commit or stash them.
Traceback (most recent call last):
...

The issue is that the stack trace is quite long, obscuring the user error and making it look like a tool failure.