feat: improve response error codes

[glennr]

1. This library only currently supports specific 400/410 errors with InvalidSubscription
2. For all other errors its difficult to determine what type of error occurred as ResponseError only provides a stringified version of the Net/HTTPResponse.

However the Web Push Protocol defines several possible responses;

https://developers.google.com/web/fundamentals/engage-and-retain/push-notifications/web-push-protocol#response_from_push_service

This PR adds the following first-class error types;

• ExpiredSubscription (404)
• PayloadTooLarge (413)
• TooManyRequests (429)

It also adds the original Net/HTTPResponse error as an attribute on ResponseError.

[glennr]

Just realized I only had spec coverage in the GCM section - whereas I thought I was adding it under VAPID - which is the protocol linked above.

I haven't tested the GCM stuff, but even if the GCM protocol doesn't work the same, each of these more specific error codes is still a ResponseError under the hood - so any existing code relying on that error class shouldn't break.

[zaru]

@glennr Thanks for PR! Great😀

[zaru]

I released v0.3.2
https://rubygems.org/gems/webpush/versions/0.3.2