gsocket: Don't g_error() if file-descriptor is not a socket

gsocket: Don't g_error() if file-descriptor is not a socket

This code was out of date with current coding practices.

Nowadays it's common to receive file descriptors over environment
variables from other processes like systemd. The unit files that
control these file descriptors are configurable by sysadmins.

It is not (necessarily) a programmer error when g_socket_details_from_fd()
is called with a file descriptor that is not a socket. It can also
be a system and/or configuration error.

https://bugzilla.gnome.org/show_bug.cgi?id=746339