night-light-dialog: Ignore G_IO_ERROR_CANCELLED errors

We should ignore G_IO_ERROR_CANCELLED errors and avoid displaying useless
warnings. Also use g_clear_object instead of g_object_unref.

Fixes: GNOME/gnome-control-center#87
2 jobs for fix-issue-87 in 11 minutes and 46 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Build
passed build #32146

00:05:55

 
  Test
passed test #32147

00:05:51