Clarify example in "Error handling"

[crandmck]

Comment in #824 :

let's follow up with some fixes to this example in general. For example, that comment right above is very confusingly worded.

[ryhinchey]

@dougwilson per your comment here https://github.com/expressjs/expressjs.com/pull/824#issuecomment-310804420, is there still an issue on the site? If so, mind adding a little context here so someone might be able to pick it up?