Fix inconsistent status codes

HTTP 401 should be used when logging in (i.e. authenticating) would make
a difference; HTTP 403 is reserved for requests that fail because the
already authenticated user is not authorized (i.e. lacking permissions)
to do something.

src/Foundation/ErrorServiceProvider.php

@@ -31,6 +31,7 @@ class ErrorServiceProvider extends AbstractServiceProvider
 
                 // 401 Unauthorized
                 'invalid_access_token' => 401,
+                'not_authenticated' => 401,
 
                 // 403 Forbidden
                 'forbidden' => 403,