Wouldn't this be useful to actually export such that error returns can be compared against it? (ie, `if err != user.RangeError ...`)

Also, the stdlib names errors more like `ErrSomething` (see http://golang.org/pkg/os/#pkg-variables for an example), so I think `ErrRange` would be better.  Also, it feels wrong to hard code the maximum value in this string when it could be sourced from our variable below (which should really be a const, and IMO we ought to have an explicit const for min, too).