Client example changes

1. client/example: Port number parameter message

Provide the error message if the port number parameter is not provided.

2. client/example: use uniform coding style for verify macro

Change the MONGO_verify macro usage to verify so as to make the examples make use of the uniform coding style

3. client/example: cursor check and return values

Check for the valid cursor before moving forward and add the correct return values for the application exit.