Improve `ResponseError`

[AlexWayfer]

• Make ResponseError#data public. It's useful for checks by fields inside rescue, re-raise or not.
• Remake specs of ResponseError#error_code.
• Add specs for ResponseError#to_s.
• Replace double quotes for any value inside ResponseError#to_s with .inspect: now the type of value is more obvious (Boolean, Integer, String, etc.).
• Improve ResponseError initialization. Don't expect basic message, have it inside the class.