Decrypts a raw push message.

This accepts the content of a Push Message (from websocket or via Native Push systems).

Arguments:

• payload: The Push payload as received by the client from Push.

Returns

Decrypted message body

Errors

Returns an error in the following cases:

• The PushManager does not contain a valid UAID

• There are no records associated with the UAID the `PushManager` contains

• An error occurred while decrypting the message

• An error occurred accessing the PushManager's persisted storage