The Classroom add-ons API may return add-ons specific PERMISSION DENIED HTTP
403 errors if an end user doesn't meet the prerequisites for access. The message
accompanying the error contains an error message to help you identify the
cause and direct users to take the appropriate action.
InvalidAddOnToken
InvalidAddOnToken indicates that the add-on token passed in a request is not
authorized to create an add-on attachment on the assignment.
Possible Action: This error can be thrown if the user signs in to the add-on with a different account than the account in Classroom. Instruct the user to either sign out of all other accounts in the browser or open Classroom in an incognito Chrome window.
ExpiredAddOnToken
ExpiredAddOnToken indicates that the add-on token being used to make calls to
the API has expired.
Possible Action: Prompt the user to refresh the page or sign in to the add-on
again so that you may obtain the new addOnToken query parameter from the
request URL.