webhooks: Use 200 status code for unknown events.

Because the third party might not be expecting a 400 from our
webhooks, we now instead use 200 status code for unknown events,
while sending back the error to Sentry. Because it is no longer an error
response, the response type should now be "success".

Fixes #24721.

(cherry picked from commit 84723654c8)

zerver/lib/exceptions.py

@@ -370,6 +370,7 @@ class UnsupportedWebhookEventTypeError(WebhookError):
     """
 
     code = ErrorCode.UNSUPPORTED_WEBHOOK_EVENT_TYPE
+    http_status_code = 200
     data_fields = ["webhook_name", "event_type"]
 
     def __init__(self, event_type: Optional[str]) -> None:
@@ -378,7 +379,9 @@ class UnsupportedWebhookEventTypeError(WebhookError):
 
     @staticmethod
     def msg_format() -> str:
-        return _("The '{event_type}' event isn't currently supported by the {webhook_name} webhook")
+        return _(
+            "The '{event_type}' event isn't currently supported by the {webhook_name} webhook; ignoring"
+        )
 
 
 class AnomalousWebhookPayloadError(WebhookError):