Add a Socket implementation which just holds an error message.

Add a Socket implementation which just holds an error message.

This isn't yet used, but I plan to use it in situations where you have
to report errors by returning a valid Socket on which the client wlil
call sk_socket_error, but in fact you notice the error _before_
instantiating your usual kind of Socket. The resulting Socket is
usable for nothing except reading out the error string and closing it.

[originally from svn r10065]