Warn early on unsupported repos #326
Description
sourcetool cannot be used at present with private repos unless they have a paid plan, this is what you get when trying to set them up:
Error: configuring controls: creating policy for: getting branch controls: checking status: GET https://api.github.com/repos/deobieta/slsa/rules/branches/main: 403 Upgrade to GitHub Pro or make this repository public to enable this feature. []
The setup and status subcommands should warn about this before doing anything else to handle onboarding (or the failure to onboard) ina more friendly way.