Add custom exception for error responses

[sunny]

Hi Makerbot team and happy new year! ✨

This fixes a TODO in the code to raise a custom exception when there is an unhandled HTTP response error, allowing for more easy debug and understanding.

This also fixes when the gem would raise "#{response.code}: #{JSON.parse(response.body)['error']}" without testing if the body is in JSON or not, therefore raising a JSON::ParserError instead of a helpful error message.