Better error message when invalid HTTP_PROXY is set

[kjk]

When HTTP_PROXY env variable is set, Go uses that for HTTP requests.
If that is invalid or proxy server fails, we return rather unhelpful error message:

Tried to send *ravendb.GetDatabaseTopologyCommand request via GET http://live-test.ravendb.net/topology?name=demo to all configured nodes in the topology, all of them seem to be down or not responding. I've tried to access the following nodes: ' of type *ravendb.AllTopologyNodesDownError

We kind of lie (we didn't try to connect to http://live-test.ravendb.net but to whatever HTTP_PROXY was set to) and it's hard to diagnose (stumped me for a while).