patch 8.2.2488: json_encode() gives generic argument error

Problem:    json_encode() gives generic argument error.
Solution:   Mention the type that can't be encoded. (issue #7802)
diff --git a/src/errors.h b/src/errors.h
index 9b9a782..aaf7e55 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -357,3 +357,5 @@
 	INIT(= N_("E1159: Cannot split a window when closing the buffer"));
 EXTERN char e_cannot_use_default_for_variable_arguments[]
 	INIT(= N_("E1160: Cannot use a default for variable arguments"));
+EXTERN char e_cannot_json_encode_str[]
+	INIT(= N_("E1161: Cannot json encode a %s"));