Better handling of connection failures in client

[DrLex0]

There is no way to explicitly catch connection failures in client code. The implementation in client/client.cc intercepts exceptions during connect, by means of its ‘ExceptionPrinter’. This prints the exception on std::cout and leaves the PromiseHttp::Response in a limbo state, meaning the calling program cannot know what exactly went wrong. It can only rely on a default error state and a timeout.

For instance, if I run the http_client.cc example with a resource path starting with a local IP address that is serving nothing (e.g. http://127.0.0.1:4321). It will print: “Got exception: Could not connect: Resource temporarily unavailable” and then wait for the 5-second barrier timeout even though it is obvious that the request has already failed.
I have tried modifying the example by defining other handlers than Async::IgnoreException, but it makes no difference. I cannot manage to trigger a lambda that takes an exception_ptr, it is never invoked.

[timlyo]

It seems that Pistache can now run a lambda in response to connection errors:

response.then([&](Http::Response response) {
                      std::cout << "Successful";
                  },
                  [](std::exception_ptr e) {
                      try {
                          std::rethrow_exception(e);
                      } catch (const Pistache::Error &e) {
                          std::cerr << e.what();
                      }
                  })

I haven't tried cancelling the timeout from this though so I'm not sure if that works.