EventIndex: Don't process checkpoints that don't have a valid token.

Some rooms might have null as a token, don't add those to be crawled nor
to the index.

Catch any other errors that might occur during the addition of an
initial checkpoint and log them.

src/indexing/EventIndex.js

@@ -115,10 +115,20 @@ export default class EventIndex extends EventEmitter {
                 direction: "f",
             };
 
+            try {
+                if (backCheckpoint.token) {
                     await indexManager.addCrawlerCheckpoint(backCheckpoint);
-            await indexManager.addCrawlerCheckpoint(forwardCheckpoint);
                     this.crawlerCheckpoints.push(backCheckpoint);
+                }
+
+                if (forwardCheckpoint.token) {
+                    await indexManager.addCrawlerCheckpoint(forwardCheckpoint);
                     this.crawlerCheckpoints.push(forwardCheckpoint);
+                }
+            } catch (e) {
+                console.log("EventIndex: Error adding initial checkpoints for room",
+                            room.roomId, backCheckpoint, forwardCheckpoint, e);
+            }
         }));
     }