LibCore: Add support for LocalServer to propogate accept() errors

We still log the error (perhaps in the future, we will only want to log
the error if there is no handler). But this allows callers to actually
handle errors to e.g. unblock waiters.

Userland/Libraries/LibCore/LocalServer.h

@@ -24,6 +24,7 @@ public:
     ErrorOr<NonnullOwnPtr<Stream::LocalSocket>> accept();
 
     Function<void(NonnullOwnPtr<Stream::LocalSocket>)> on_accept;
+    Function<void(Error)> on_accept_error;
 
 private:
     explicit LocalServer(Object* parent = nullptr);