So I'm failing to find any documentation for libedit, but it looks as though this error occurs if rl_read_init_file fails to find an .editrc file in the appropriate place. If I create an empty .editrc file in my home directory, the error disappears. (Having an .inputrc file doesn't seem to make a difference either way.)
Perhaps the solution is to ignore an OSError from readline.read_init_file in register_readline---i.e., add a try / except OSError there. The other option would be to ignore a nonzero errno from rl_read_init_file, but that doesn't sound so good: I'd expect someone deliberately calling readline.read_init_file to get an exception if the initialization file isn't found.