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)

src/errors.h

@@ -3447,3 +3447,5 @@ EXTERN char e_cannot_define_new_function_in_abstract_class[]
 EXTERN char e_using_null_object[]
 	INIT(= N_("E1360: Using a null object"));
 #endif
+EXTERN char e_cannot_use_color_none_did_you_mean_none[]
+	INIT(= N_("E1361: Cannot use color \"none\", did you mean \"NONE\"?"));

src/gui.c

@@ -4634,12 +4634,18 @@ gui_get_color(char_u *name)
 	return INVALCOLOR;
     t = gui_mch_get_color(name);
 
+    int is_none = STRCMP(name, "none") == 0;
     if (t == INVALCOLOR
 #if defined(FEAT_GUI_X11) || defined(FEAT_GUI_GTK)
-	    && gui.in_use
+	    && (gui.in_use || is_none)
 #endif
 	    )
-	semsg(_(e_cannot_allocate_color_str), name);
+    {
+	if (is_none)
+	    emsg(_(e_cannot_use_color_none_did_you_mean_none));
+	else
+	    semsg(_(e_cannot_allocate_color_str), name);
+    }
     return t;
 }
 

src/testdir/test_highlight.vim

@@ -46,6 +46,10 @@ func Test_highlight()
   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)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1328,
 /**/
     1327,
 /**/