According to Baseball Reference, since 1876, there have been 499,590 errors made. So only 410 to go. With 30 teams and about 40 games left for each, there are close to 1200 games left in the season. With a rate of .5-.6 errors per game, there should be around 600 more errors this season and we will pass the 500,000 mark.
There have also been 98,790 hit batters, so that could be passed in a couple of years.
Update: Jose Reyes made the 500,000th error on Sept. 15th. See Marlins' Jose Reyes fumbles way into history with baseball's 500,000th error by Jeff Passan. It also has a video clip.