Check connection to the database

If the database doesn't exist, the following exception occured:
```
pymongo.errors.ServerSelectionTimeoutError: localhost:27017: [Errno 61] Connection refused
```

Need to provide database check during initialization and catch the error

Locate the initialization code that creates the MongoDB connection and reproduce the missing-database case on localhost:27017. Trace the pymongo connection failure, then verify that initialization performs the requested check and handles ServerSelectionTimeoutError without an unhandled exception.

Written by the indexing model from the issue text.