Upgraded estimator rescue log level to Error

Per a discussion in #spree I feel that the `.error` log level is more appropriate here - something bad's happening and while we definitely want to recover and not return a `500` we still need to give the admin a heads up ASAP.  In the absence of a critical error detection meta-framework the `error` loglevel seems like the way to go.  I can set up an email alert on this loglevel in Papertrail for instance.