Improve database connection error handling

[mattstibbs]

Currently, if the db_connector can't make a connection to the database on startup, a fatal exception is thrown. This may be related / intertwined with the audit_logger module (which attempts to get a database connection) - separate issue has been raised for this.

Need to improve the database connection logic so that a useful error is provided back when the database connection is not possible.

Also potentially implement some retry logic?