When updating the config, keep the previous file if the new one is malformed

[thibaultamartin]

Hebbot crashes when the configuration file is not properly formatted, which can happen quite easily (e.g. when putting the value of the order field between single quotes instead of nothing, or when omitting the usual_reporters field).

It would make Hebbot a lot more solid if it kept a previous version of the config file and performed a rollback in case of crash: without it, there's a need for a technical administrator to manually update the config file instead of just running !update-config after correcting the issue.

Alternatively, Hebbot could run the update_config_command when it starts.