Error handling for ARI "replaces" conflict

[webprofusion]

Yes, agree it maybe need more explicit definition as noted on my previous comments on ARI implementations - #8 by webprofusion

@webprofusion

For those implementing ARI, things to watch out for include:

• an invalid replaces certID (wrong format, not the current cert id, previous cert is issued from a different CA etc) will cause the order to error with a Conflict status code [this response could vary between implementations]. Catch your errors and consider skipping "replaces" if you've had a few repeated failures, otherwise you could end up in a self imposed denial of service by way of an unforeseen combination of circumstances.

I feel that "replaces" is a courtesy to the CA, so failing the order seems unnecessary and error prone.