Client example changes

[PATCH 1/3] client/example: Port number parameter message

Added check to provide the error message if the port number parameter is not provided.

[PATCH 2/3] client/example: use uniform coding style for verify macro

This patch changes the MONGO_verify macro usage to verify so as to make the examples make use of the uniform coding style

[PATCH 3/3] client/example: cursor check and return values

This patch add the check for the valid cursor before moving forward and add the correct return values for the application exit.