This is essentially #59 with test.

Whenever there is an `Errno::ECONNREFUSED error`, Faraday wraps it in a
`Faraday::ConnectionFailed` error. Since it inherits `Faraday::ClientError`
without a response (see errors/faraday_server_error.rb), the code would return
a `NoMethodError: undefined method []' for nil:NilClass` error.

So basically the fix is to rescue `Faraday::ConnectionFailed` before rescuing
`Faraday::ClientError` so that we can raise a more meaningful error.

Result:
```
Graphlient::Errors::ConnectionFailedError: Failed to open TCP connection to
localhost:3000 (Connection refused - connect(2) for "localhost" port 3000)
```