Handle non-json errors #29

Hello, I am getting the following error:
TypeError Cannot access offset of type string on string
from the file:
lib/ConvertApi/Client.php:156

This is because the server returns a non-json string.
This case should throw an Error\Api just like a json-error.
Just add this before using the $json as an array, on line 156:
if (is_string($json)) throw new Error\Api($json);