return error if no subreddit is invalid

src/main/java/sh/adb/RandomRedditMemesAPI/CommunityHandler.java

@@ -63,7 +63,10 @@ class CommunityHandler implements HttpHandler {
         try {
             submissions = this.api.getSub(pathArray[4]);
         } catch (RedditParseException e) {
+            responseJSON.put("error", "invalid subreddit");
+            sendResponse(t, responseJSON.toString());
             e.printStackTrace();
+            return;
         }
         assert submissions != null;