TEST: Proxy Failover Scenarios

This test cases uses [WireMock](http://wiremock.org/index.html) to simulate various proxy failure scenarios and verify CVMFS's failover behaviour. WireMock needs a bit of setting up, though. I'll adapt the cloud platform contextualisation scripts soon, to allow the test case to run properly.

Also this adds `CURLE_GOT_NOTHING` and `CURLE_RECV_ERROR` to the error handling in the download module.