g_file_copy: Don't set GError when we intend to ignore errors

For better or worse, the current g_file_copy intention was to ignore
errors copying metadata, but we still set the GError, while returning
TRUE.

https://bugzilla.gnome.org/show_bug.cgi?id=727559

gio/gfile.c

@@ -3252,7 +3252,7 @@ file_copy_fallback (GFile                  *source,
                                        info,
                                        G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS,
                                        cancellable,
-                                       error);
+                                       NULL);
     }
 
   g_clear_object (&info);