[permissions] Specify how to behave when an extended Permission dictionary doesn't match requirements (#14)

For example, if there is the following permission associated with the name _quota_:
```js
enum QuotaType {
  "persistent",
  "temporary"
};
dictionary QuotaPermission : Permission {
  required unsigned long long quota;
                QuotaType type = "persistent";
};
```

What should happen if this call is made:
```js
Permissions.query({'name': 'quota'});
```

I believe it should fail the same way as if the name was unknown but WebIDL doesn't provide us with that failure for free.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/permissions/issues/14