Track the number of failures, not the instances.
Track the number of failures, not the instances.

The number of failures on very large scans (1m hosts) results in
a large list. This commit tracks the number of failures instead
of the objects.

0xPoly/ooni-probe,kdmurray91/ooni-probe,0xPoly/ooni-probe,lordappsec/ooni-probe,lordappsec/ooni-probe,juga0/ooni-probe,lordappsec/ooni-probe,kdmurray91/ooni-probe,juga0/ooni-probe,0xPoly/ooni-probe,juga0/ooni-probe,kdmurray91/ooni-probe,kdmurray91/ooni-probe,0xPoly/ooni-probe,lordappsec/ooni-probe,Karthikeyan-kkk/ooni-probe,juga0/ooni-probe,Karthikeyan-kkk/ooni-probe,Karthikeyan-kkk/ooni-probe,Karthikeyan-kkk/ooni-probe