REST interface from oauth2-server errors in nodejs

Question

I am using the oauth2-server plugin for nodejs (npm) and have the following configuration:

app.oauth = oauthserver({
    model: oauth.model,
    grants: ['password'],
    debug: false
});

When I GET /oauth/token without sending any parameter with (no header, no query, no body), I get the following error as an 500 - Internal Server Error:

{
    "name": "OAuth2Error",
    "message": "Invalid or missing grant_type parameter",
    "headers": {
        "Cache-Control": "no-store",
        "Pragma": "no-cache"
    },
    "code": 400,
    "error": "invalid_request",
    "error_description": "Invalid or missing grant_type parameter"
}

The code in the message says 400 (Bad Request) and I want to send the message not as a 500 but 400 response.

Additionally I get the error on my console even though I have debug: false as configuration. How can I change that?