Refactoring MONGO_METHOD calls in mongoclient.c and mongo.c

https://jira.mongodb.org/browse/PHP-820 and related sub-tasks
This was adapted from `PHP_MONGO_GET_LINK`, which is:

```
#define PHP_MONGO_GET_LINK(obj) \
	link = (mongoclient*)zend_object_store_get_object((obj) TSRMLS_CC); \
	MONGO_CHECK_INITIALIZED(link->servers, Mongo);

#define MONGO_CHECK_INITIALIZED(member, class_name) \
	if (!(member)) { \
		zend_throw_exception(mongo_ce_Exception, "The " #class_name " object has not been correctly initialized by its constructor", 0 TSRMLS_CC); \
		RETURN_FALSE; \
	}
```

The only reason I reproduced it here is because `RETURN_FALSE` wasn't appropriate in this context.