LibCore+LibHTTP: Check the status of the socket after EINPROGRESS

Previously the system would assume the socket was connected after the
file descriptor became writeable. Just because the fd is signaled as
ready for output does not necessarily indicate the socket is connected.
Instead, we should check the status of the socket with SO_ERROR and
handle successes/errors accordingly.

Userland/Libraries/LibCore/Socket.h

@@ -43,6 +43,7 @@ public:
     int destination_port() const { return m_destination_port; }
 
     Function<void()> on_connected;
+    Function<void()> on_error;
     Function<void()> on_ready_to_read;
 
 protected: