[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.
this is a quite artificial test mocking the expected exception to see if the wrapping works.

unfortunately the MockPlugin for guzzle only supports CurlException and no other classes of exceptions, limiting us to this brute force mocking...