Add ability to re-throw errors

[sam-at-luther]

We lose stack traces using the current pattern of rethrowing errors in handler-bind, e.g.:

                      (handler-bind ([condition ; catch any error
                                     (lambda (c &rest args)
                                       (log-metrics)
                                       (apply error c args))]) ; re-throw the error

We would like the ability to "re-throw" the error within this handler-bind.