fix(messaging): Throw exception if APNS token is not yet available

[russellwheatley]

Description

The Apple event handler that receives the APNS token may not have been called before the User starts to make API requests such as getToken() & subscribeToTopic(). We have set an exception when it isn't set and also updated documentation to reflect the requirement.

Related Issues

closes #10625
closes #10656

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Breaking Change

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[russellwheatley]

hey @kevinthecheung - documentation update that could do with a review, please 😄

[msarkrish]

@russellwheatley I have one doubt. I'm getting this exception, While calling subscribeToTopic method in iOS.
My question is it mandatory to make call getAPNSToken() method before calling subscribeToTopic method?

Or What I have to do to avoid this exception?