patch 8.2.3818: cannot filter or map characters in a string

Problem:    Cannot filter or map characters in a string.
Solution:   Make filter() and map() work on a string. (Naruhiko Nishino,
            closes #9327)
diff --git a/src/errors.h b/src/errors.h
index 452ff45..156e194 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -758,3 +758,5 @@
 	INIT(= N_("E1248: Closure called from invalid context"));
 EXTERN char e_highlight_group_name_too_long[]
 	INIT(= N_("E1249: Highlight group name too long"));
+EXTERN char e_argument_of_str_must_be_list_string_dictionary_or_blob[]
+	INIT(= N_("E1250: Argument of %s must be a List, String, Dictionary or Blob"));