notification: Make a bit of fromAPNsImpl's jsdoc more precise.

As Greg says [1],

> Probably "interesting" wasn't a very clear way to write that
> description; the distinction that I think it's intended to get at
> is "unexpected", as in "buggy".

[1] #4163 (comment)

src/notification/extract.js

@@ -97,7 +97,7 @@ class ApnsMsgValidationError extends Error {
 // APNs notification.
 //
 // @returns A `Notification` on success, `undefined` on suppressible failure.
-// @throws An ApnsMsgValidationError on interesting failure.
+// @throws An ApnsMsgValidationError on unexpected failure.
 //
 export const fromAPNsImpl = (rawData: JSONableDict): Notification | void => {
   /** Helper function: fail. */