Add missing curl error and return json data

Hello Christian,

I suggest returning a raw data instead of the json_decoded one, that is especially due to the fact,
that in case where the result was paginated the data obtained will not work with php serialization, making it hard to cache the result, for example with the usage of WordPress set_transient and get_transient functions. 

Moreover leaving the data in json_encoded format is also desired in case of ajax calls, that make usage of the api and require json_encoded data.

I have also added a missing curl_error info.

Regards,
Rafal