🚸(front) add more context if available about message delivery status

We recently display recipient delivery status tooltip when the message is not
yet delivered or in failure. To help user to understand why the message is not
delivered, we now display the delivery message if there is one.

src/backend/core/api/openapi.json

@@ -6351,6 +6351,11 @@
                         "readOnly": true,
                         "nullable": true
                     },
+                    "delivery_message": {
+                        "type": "string",
+                        "readOnly": true,
+                        "nullable": true
+                    },
                     "retry_at": {
                         "type": "string",
                         "format": "date-time",
@@ -6367,6 +6372,7 @@
                 "required": [
                     "contact",
                     "delivered_at",
+                    "delivery_message",
                     "delivery_status",
                     "retry_at"
                 ]