Add execption handler for guzzle_json_decode

[emsyzz]

If request fails - \Contentful\Core\Api\Requester::createCustomException is called, which assumes that response body will always be valid JSON string - but unfortunately in rare cases it isn't. Invalid response body passed to guzzle_json_decode results in InvalidArgumentException witch MUST be caught otherwise it propagates up the frame and source of original exception no longer can be determined.

This Pull Request adds additional exception handling for cases when guzzle_json_decode fails and adds additional exception class for such cases.

[pgrigoruta]

Thanks @emsyzz ! We just merged your PR into the latest release and added one coverage test.

[emsyzz]

Thank you @phoebeschmidt for such fast response. When I could expect those changes to be incorporated with management library?
I'm currently using 2.0 version of management library. Is version 2.x still being supported or I should move to 3.x?

[pgrigoruta]

@emsyzz , I don't see this code being duplicated in the management library, so you should be fine by running composer update to pull a new version of the core, when on 3.x.

I strongly encourage to upgrade to 3.x. The "breaking" change that happened in 3 is that we added support for PHP 7.4, dropped support for PHP 7.0 and 7.1 and also upgraded all deps. Unless you are on PHP 7.0 or 7.1, the upgrade will be smooth.

We don't currently plan to support 2.x anymore as generally we try to support the PHP versions that are not eol and between 7.1 and 7.4 we can have one or another.