`reflex package` should do verification that repo exists before pulling down zip

Currently if a user adds a rule that is misnamed or doesn't exist to their `reflex.yaml`, `reflex package` fails with a cryptic "FileIsNotAZip" error (going from memory so that's not the exact exception).

We need to add some verification to `reflex package` that can detect if the repo it is trying to pull down a zip from exists and is valid, and if not provide more user friendly error messages.