[P3] payment_stripe: Exception.message removed

Apparently missed in 07ab8b6cd2

In Python 3, there is no Exception.message attribute anymore, this would
lead to cascading exceptions in the stripe_s2s_create handler.

closes odoo/odoo#28141

addons/payment_stripe/controllers/main.py

@@ -27,7 +27,7 @@ class StripeController(http.Controller):
         try:
             acquirer.s2s_process(post)
         except Exception as e:
-            error = e.message
+            error = str(e)
 
         return_url = post.get('return_url', '/')
         if error: