outgoing_webhook: Simplify error handling logic.

Structurally, exception, failure_message, and status_code are mutually
exclusive in how this function is called, and it's best for the
function's flow to represent that.

zerver/lib/outgoing_webhook.py

@@ -241,23 +241,23 @@ def notify_bot_owner(
     assert bot_owner is not None
 
     notification_message = f"[A message]({message_url}) to your bot @_**{bot.full_name}** triggered an outgoing webhook."
-    if failure_message:
+    if exception:
+        notification_message += (
+            "\nWhen trying to send a request to the webhook service, an exception "
+            f"of type {type(exception).__name__} occurred:\n```\n{exception}\n```"
+        )
+    elif failure_message:
         notification_message += "\n" + failure_message
-    if status_code == 407:
+    elif status_code == 407:
         notification_message += (
             "\nThe URL configured for the webhook is for a private or disallowed network."
         )
     elif status_code:
         notification_message += f"\nThe webhook got a response with status code *{status_code}*."
-    if response_content and status_code != 407:
+        if response_content:
             notification_message += (
                 f"\nThe response contains the following payload:\n```\n{response_content!r}\n```"
             )
-    if exception:
-        notification_message += (
-            "\nWhen trying to send a request to the webhook service, an exception "
-            f"of type {type(exception).__name__} occurred:\n```\n{exception}\n```"
-        )
 
     message_info = dict(
         type="private",