patch 9.0.1328: error when using "none" for GUI color is confusing

Problem:    Error when using "none" for GUI color is confusing.
Solution:   Mention that the name should perhaps be "NONE". (closes #1400)
diff --git a/src/testdir/test_highlight.vim b/src/testdir/test_highlight.vim
index 26426e0..feccf84 100644
--- a/src/testdir/test_highlight.vim
+++ b/src/testdir/test_highlight.vim
@@ -46,6 +46,10 @@
   call assert_equal("Group3         xxx cleared",
 				\ split(execute("hi Group3"), "\n")[0])
   call assert_fails("hi Crash term='asdf", "E475:")
+
+  if has('gui_running')
+    call assert_fails('hi NotUsed guibg=none', 'E1361:')
+  endif
 endfunc
 
 func HighlightArgs(name)