Minor Bug in Update and Delete Api

[rm210]

Minor bug in Update and Delete API Created. The bug I faced is that when we execute the API's like update and delete in Postman Software for the query 'DELETE FROM ' . $this->table . ' WHERE id = :id' when the "id" is not in the Database still the Query executes and does not throw any error.
So, i wrote a Check() function in Post.php which will check if any entry with following "id" is present or not and will throw an error accordingly.

I will be contributing tomorrow.