Add `command_err_badjson` helper for sync error handling, mirroring the async `command_err_bcli_badjson`. Store args string in `bcli_result` for consistent error messages.
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).