Show helpful error when oplog is missing

app/models/server/models/_BaseDb.js

@@ -44,6 +44,14 @@ export class BaseDb extends EventEmitter {
 				collection: this.collectionName,
 			};
 
+			if (!MongoInternals.defaultRemoteCollectionDriver().mongo._oplogHandle) {
+				throw new Error(`Error: Unable to find Mongodb Oplog. You must run the server with oplog enabled. Try the following:\n
+				1. Start your mongodb in a replicaset mode: mongod --smallfiles --oplogSize 128 --replSet rs0\n
+				2. Start the replicaset via mongodb shell: mongo mongo/rocketchat --eval "rs.initiate({ _id: ''rs0'', members: [ { _id: 0, host: ''localhost:27017'' } ]})"\n
+				3. Start your Rocket.Chat instance with OPLOG configuration: export MONGO_OPLOG_URL=mongodb://localhost:27017/local node main.js
+				`);
+			}
+
 			MongoInternals.defaultRemoteCollectionDriver().mongo._oplogHandle.onOplogEntry(
 				query,
 				this.processOplogRecord.bind(this)