Setting `use_ssl` to false without changing `ssl_options` causes a crash

[dantswain]

From this section of the config:

  # This is the flag that enables use of ssl
  use_ssl: true,
  # see SSL OPTION DESCRIPTIONS - CLIENT SIDE at http://erlang.org/doc/man/ssl.html
  # for supported options
  ssl_options: [
    cacertfile: System.cwd <> "/ssl/ca-cert",
    certfile: System.cwd <> "/ssl/cert.pem",
    keyfile: System.cwd <> "/ssl/key.pem",
  ],

I would expect that setting use_ssl to false will simply make ssl_options irrelevant. Instead it causes the application to fail to start with a

** (Mix) Could not start application kafka_ex: KafkaEx.start(:normal, []) returned an error: :badarg

which is pretty unhelpful.

I propose:

1. If use_ssl is set to false, the application should ignore ssl_options.
2. If anything in ssl_options is configured incorrectly, the application should raise a meaningful error to guide the user in determining what is wrong.