Finishes handling a D-Bus method call by returning an error.
See encode_gerror for details about what error name will be returned on the wire. In a nutshell, if the given error is registered using register_error the name given during registration is used. Otherwise, a name of the form `org.gtk.GDBus.UnmappedGError.Quark...` is used. This provides transparent mapping of Error between applications using GDBus.
This method will take ownership of this. See DBusInterfaceVTable for more information about the ownership of this.
Since 2.48, if the method call requested for a reply not to be sent then this call will free this but otherwise do nothing (as per the recommendations of the D-Bus specification).
The error code.