[WIP] testing guzzle exceptions

i tried to trigger the guzzle exception handling we have. [code coverage](https://scrutinizer-ci.com/g/FriendsOfSymfony/FOSHttpCache/code-structure/master/class/FOS%5CHttpCache%5CProxyClient%5CAbstractProxyClient) indicates we never see a RequestException in the handleException method. however, i was not able to ever produce that exception. any ideas what we could do? or can we simply drop that excpetion? or am i just confused?

nevermind the code in this PR, i have left all kind of debugging and experimenting stuff lying around.
not sure if this really is a good idea, but saw that this is the base class. the ClientInterface does not explain what exceptions to expect.