Handling bad cn=config updates

If I modify olcDbDirectory of a BDB database to a broken directory, the Modify returns success but BDB says "failed to reopen database" and slapd shuts down. Maybe other modifications can shut down slapd too, I don't know.

It would be nice if slapd tried to restore the old config value, reopen the old directory, and return unwillingToPerform or something to the Modify. However I don't know how hard that would be, or how obscure a case it is.

For that matter, maybe if the Modify broke something it could return some diagnosticMessage. Not sure if it should return non-success since that supposedly means the modify did not happen. Though clients may not display the diagnosticMessage if it comes with 'success'.