I'm pretty sure it's just saying that there will always be an error / threat range - there is no rule that just "removes" the error range. So having a base error of 0 sounds like it's gone, but there will be situations (firing a gun after it's been in the mud) that might give it a +X (0 != nothing, therefore 0 + X != nothing). If that makes sense. Same thing in reverse - if your threat range is 22, but you have a trick that gives +2 threat range to that attack then it becomes 20 again (and not "nothing" followed by either "nothing still" or "18" or "19" - depending on how you do the maths).
paddyfool isn't technically correct - the column opposite on the same page outright states that if it's a success, it's not an error.
When a skill check fails and the character rolls a natural number within his error range (an actual roll of the number on a d20), or when a skill result is negative, the character suffers an error.
However, myself (and many others) like having a success also be an error due to the narrative potential ("Your shot kills your target, but it's a through and through and wounds a bystander" // "You deactivate the trap, but the noise of doing so alerts the guards next door"). So I personally totally agree with him (as do many
others), but just want to clarify that it's not the RAW (though I hope that changes for SC3 and 10k Bullets - it's too cool to not have).