Make ClientInterface implementations throw an exception on non 2xx HTTP response codes

[aik099]

The current implementation of ClientInterface has several issues:

• not all error HTTP codes are converted to an exception (e.g. HTTP 401 code is, but others are not)
• the API response is lost when exception is thrown
• cURL transport errors (for CurlClient only) aren't handled at all

Proposing to:

1. throw an exception on any non-2xx HTTP response code
2. for the \chobie\Jira\Api\Exception class:
   • add the protected rawResponse property, which will be populated from the constructor
   • add public getRawReponse method, that will return the above property
   • add public getResponse method, that will return a json-decoded version of the raw response

This way a developer will be informed about the error (through an exception being thrown) and can do further error analysis using the getResponse method.