throw an error if getSwarmNodes cannot find the conversation

js/modules/loki_snode_api.js

@@ -772,6 +772,9 @@ class LokiSnodeAPI {
     const { fetchHashes } = options;
     try {
       const conversation = ConversationController.get(pubKey);
+      if (!conversation) {
+        throw new Error('Could not find conversation ', pubKey);
+      }
       const swarmNodes = [...conversation.get('swarmNodes')];
 
       // always? include lashHash