I'm assuming these throws were coming from the catcher on a play where the runner at first was attemting to steal.
#1 Fortunately or unfortunately, depending on your perspecitve, the rules do not allow an error to be credited for a mental error (which it sounds like this was). And if the throw was good, you can't give the catcher an error. So about all you can do is give the runner a stolen base and not assign an error on the play.
#2 OBR 10.08 (f) states:
"When in the scorer's judgment a runner attempting to steal is safe because of a muffed throw, do not credit a stolen base. Credit an assist to the fielder who made the throw; charge an error to the fielder who muffed the throw, and charge the runner with "caught stealing."
This discussion has come up previously on this thread, and experienced scorekeepers have stated that for this rule to apply it has to be extremely obvious that the runner would have been out had the fielder not "muffed the throw".
The way you've described it, if I were keeping the books I'd apply this rule -- but bottom line it's a scorekeeper's judgement call.
Hope that helps.