Return false when no connection can be made

Currently when an ezSQL class is instantiated, it doesn't open a connection to the server. This connection is created on the fly, if needed, when function `query()` is called, but no additional checks were made to make sure there was an actual connection.

As a result, various `mysql_` or PDO operation were performed and causing warning or fatal error (unoticed because of the `@` operator)

This PR makes the PDO, MySQL and MySQLi return false when a `->query()` is attempted against a server that is not responding.