Fail gracefully if repo does not exists

[Carreau]

Otherwise user get the error:

Traceback (most recent call last):
  File "/Users/bussonniermatthias/anaconda/bin/doctr", line 9, in <module>
    load_entry_point('doctr', 'console_scripts', 'doctr')()
  File "/Users/bussonniermatthias/dev/doctr/doctr/__main__.py", line 198, in main
    return process_args(get_parser())
  File "/Users/bussonniermatthias/dev/doctr/doctr/__main__.py", line 92, in process_args
    return args.func(args, parser)
  File "/Users/bussonniermatthias/dev/doctr/doctr/__main__.py", line 142, in configure
    encrypted_variable = encrypt_variable(b"DOCTR_DEPLOY_ENCRYPTION_KEY=" + key, repo=repo)
  File "/Users/bussonniermatthias/dev/doctr/doctr/local.py", line 44, in encrypt_variable
    public_key = r.json()['key']
KeyError: 'key'

Because r.status is not 200...