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/errors.h b/src/errors.h
index 0bfafac..a5df8ed 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3447,3 +3447,5 @@
 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\"?"));