Improve the error message when the request fails

[jgauffin]

Can you add the response content to the exception message?
It will help immensely when getting started.

In HandleResponse:

// you might want to have a try/catch, there are cases when reading
// the content fails (although it should work in most cases)
var str = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
throw new WebPushException($"{message}. Details: {str}", response.StatusCode, response.Headers, subscription);

Before

WebPush.WebPushException: 'Received unexpected response code: 403'

After

WebPush.WebPushException: 'Received unexpected response code: 403. Details: the key in the authorization header does not correspond to the sender ID used to subscribe this user. Please ensure you are using the correct sender ID and server Key from the Firebase console.

[coryjthompson]

Thank you for the advice. This was released in version v1.0.12.