gfile: Use a more specific error message if symlinks are not supported

The string is already translated in `GLocalFile`, so this doesn’t
introduce a new translatable string.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/gfile.c

@@ -4046,7 +4046,7 @@ g_file_make_symbolic_link (GFile         *file,
     {
       g_set_error_literal (error, G_IO_ERROR,
                            G_IO_ERROR_NOT_SUPPORTED,
-                           _("Operation not supported"));
+                           _("Symbolic links not supported"));
       return FALSE;
     }