Handle validation errors in PublicBody.import_csv

Specifically using save! so that anything other than an
ActiveRecord::RecordInvalid doesn't get missed

Note that ActiveModel::Errors#full_messages includes the attribute key
in the message. This is by design, so we should consider whether we can
improve the way that we use translated validation messages.
Remove unrecognised field from import_csv example

`id` is not a recognised attribute when uploading from a CSV.