Issue Summary

When creating a subscriber for an Audience Specific page user using the API https://developer.statuspage.io/#operation/postPagesPageIdSubscribers , the error 
"An unknown error has occurred." is returned as the JSON body instead of adding the Subscriber.

Steps to Reproduce

1. Use an Audience Specific page as the target for these API calls
2. Create a Page Access User using https://developer.statuspage.io/#operation/postPagesPageIdPageAccessUsers 
3. Create a subscriber for this user, specifying individual components to be subscribed to using the Subscriber API https://developer.statuspage.io/#operation/postPagesPageIdSubscribers 

Expected Results

The page access user is subscribed to the components specified

Actual Results

The below exception is returned in the JSON response:

{
    "error": [
        "An unknown error has occurred."
    ]
} 

Workaround

First when creating the Page Access User, ensure the subscribe_to_components boolean flag is set to true. Then instead of using the Subscribe API, use the Add Components to a Page Access User API to add the components as detailed in:  https://developer.statuspage.io/#operation/patchPagesPageIdPageAccessUsersPageAccessUserIdComponents