PHP-1063: Refactor cursor logic to follow MongoCommandCursor

https://jira.mongodb.org/browse/PHP-1063
Don't we need to call php_mongo_handle_error() in case the first document was a error document?

It seems a bit inconcistent which functions are called after _runquery()
See https://github.com/mongodb/mongo-php-driver/pull/737/files#diff-61d70c33f4e01e6daaafe6855a3b76e9R905 for example
it does loads of error handling, while others just touch "the surface"...