[push-api] Mark `PushMessageData` methods as throwable (#165)

`arrayBuffer()`, `blob()`, and `json()` can throw due to, e.g., memory allocation failures or invalid JSON.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/165

Received on Thursday, 17 September 2015 12:20:19 UTC