In elegant failure in `gs4_auth` when token file does not exist

[collinschwantes]

When a path is provided to gs4_auththat does not resolve to a file, the function automatically enters interactive mode. It would be nice if the function warned you or errored out if the file does not exist at that path.

Brief description of the problem:

function enters interactive mode without signaling to user that a file doesnt exist.

key_path <- "./auth/secret.json"
gs4_auth(path = key_path)