Verify that the module paths exist

[ThunderKey]

As mentioned in #533, I'm providing a patch so that the module paths are verified to exist.

An example output of an init command where the module path (some_inexisting_file in this case) does not exist:

$ cosmic-ray init cosmic-ray.toml cosmic-ray.sqlite                                                        
Could not find module path some_inexisting_file

[abingham]

Thanks!