Bug 623143 – Never require non-closed connections

There's a couple of places in GDBus where it's a programming error
(e.g. we'll assert or spew via e.g. g_warning()) to use the API on a
closed connection.  This approach can never work since a
GDBusConnection can be closed at any point in time outside of
programmer control.

Just change the code to return a run-time error (e.g. return
G_IO_ERROR_CLOSED when sending messages, invoking methods) or silently
accept the request (e.g.  exporting objects, registering for signals)
without doing anything.

Note that a GDBusConnection object is always useless after being
closed - e.g.  there's no way to "reopen" a connection - the user will
have to create a new object and use that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=623143



Signed-off-by: David Zeuthen <davidz@redhat.com>