Errors can be errors like 404 your proxy is dead, but most often its simply blocked ips. 10 private proxies with 3 connections will guarantee your proxies will all be banned. You would want to use the detailed harvester and set a delay of probably 5 or more. As for server proxies they are heavily used, so you will see a lot of errors as they die and get blocked.
Basically you can do this. Go into your scrapebox folder >> error logs folder and look at the harvester error log. If its got a lot of 404, 500 etc... type codes, your proxies are bad, else they are just blocked. So wait till they unblock and add a delay if your not using server proxies. Don't use the detailed harvester and a delay and server proxies, that will not work out well. I don't even use public proxies, just private with a delay and if you go slow enough to no get blocked its set and forget it and you can scrape tens of thousands of results pretty fast.