Throw `WebPushStatusException` as a checked exception

[stefanrother]

Problem

WebPush.getSubscriptionState() throws a WebPushStatusException which causes problems when calling this library from Java:

• Java tools don't neccessarily recognize that a WebPushStatusException is thrown (it's more or less invisible to Java)
• in a unit test it's hard to mock an exception that Java doesn't believe to be thrown

Suggestion

Add @Throws(WebPushStatusException::class) to WebPush.getSubscriptionState()