Handle authorization errors

[hackermd]

Slim uses OIDC to authenticate users and authorize the application to access a DICOM store on behalf of the user. The authentication and authorization may be handled by two separate systems (identity and authorization provider) and there may be situations, where a user is able to successfully authenticate but then doesn't have permissions to access the DICOM store such that the application fails to search for or retrieve data. Slim should handle HTTP requests that fail due to lack of authorization (response message with status code 403).