Adds the ability to check for the errors on records after a failing operation.

It's currently hard to find out what happened when an operations fails:
```ruby
invoice = NetSuite::Records::Invoice.new({
    created_from: {
        internal_id: "<invalid ref number>"
    }
})

invoice.add # returns false. But what happened?
```

The idea is to provide an errors method to check what happened: 
```ruby
invoice.errors.any? # true
invoice.errors.first.code # "INVALID_INITIALIZE_REF"
invoice.errors.first.message # "You can not initialize invoice: invalid reference 7281."
``` 

Great gem, thanks! :beers: