python: Convert translated positional {} fields to {named} fields.

Translators benefit from the extra information in the field names, and
need the reordering freedom that isn’t available with multiple
positional fields.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/webhooks/common.py

@@ -228,4 +228,6 @@ def unix_milliseconds_to_timestamp(milliseconds: Any, webhook: str) -> datetime:
         seconds = milliseconds / 1000
         return timestamp_to_datetime(seconds)
     except (ValueError, TypeError):
-        raise JsonableError(_("The {} webhook expects time in milliseconds.").format(webhook))
+        raise JsonableError(
+            _("The {webhook} webhook expects time in milliseconds.").format(webhook=webhook)
+        )