patch 8.2.4680: build failure without +postscript

Problem:    Build failure without +postscript.
Solution:   Use another error message.
diff --git a/src/testdir/test_vim9_import.vim b/src/testdir/test_vim9_import.vim
index 5729d73..2c3792f 100644
--- a/src/testdir/test_vim9_import.vim
+++ b/src/testdir/test_vim9_import.vim
@@ -1001,7 +1001,7 @@
       delete('XimportRelDel.vim')
       DoIt()
   END
-  v9.CheckScriptFailure(lines, 'E456:')
+  v9.CheckScriptFailure(lines, 'E484:')
 
   delete('XimportRel.vim')
   delete('XimportRel2.vim')