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

[mcbanderson]

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.