cart.js: No 'new' on util.error

[amnotafraid]

If I have a bad product_id when I try to POST to /cart/add-item, I get error messages that start out like this:

TypeError: util.error is not a constructor
    at schema.get.then.product (/Users/razoyo-dev/apps2/schema-node-api/api/v1/cart.js:371:13)

This change fixes it, but do you want to do new Error(... rather than util.error(...?

[ericingram]

Looks like it was an "error" to have used the new keyword here in the first place. Fix looks good, thank you!