patch 9.0.0444: trying to declare g:variable gives confusing error

Problem:    Trying to declare g:variable gives confusing error.
Solution:   Give a better error message. (closes #11108)
diff --git a/src/errors.h b/src/errors.h
index d1cf4b4..5f0fdcb 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3335,4 +3335,6 @@
 	INIT(= N_("E1302: Script variable was deleted"));
 EXTERN char e_custom_list_completion_function_does_not_return_list_but_str[]
 	INIT(= N_("E1303: Custom list completion function does not return a List but a %s"));
+EXTERN char e_cannot_use_type_with_this_variable_str[]
+	INIT(= N_("E1304: Cannot use type with this variable: %s"));
 #endif