Fix NPE when getting error message and add HTTP response information

[jorgee]

close #3
The current implementation assumes the request will return an HTTP response with an ErrorResponse serialized as JSON, but this only happens when the publish registry URL is correct. In other cases, the error response is null or throws a JSONParseException, hiding the real reason for the error.

This PR fixes this incorrect behaviour by checking the response content.

• If content matches with ErrorResponse, the message includes HTTP code, error type, and message
• If content doesn't match with ErrorResponse, the message includes the HTTP code and the content string
• If there is no content, it just returns the HTTP code