The "cost" value was flawed for a couple of reasons.
1. Lots of messages are free, so in those instances the "cost"
doesn't tell you anything
2. The query to get the rate was expensive and we don't have
an obvious way to get it back very efficiently for large numbers
of notifications.
So we scrapped it.
This is the schema that individual notifications will conform to
when they are returned from this API.
JSON logic enforces that the right keys are set depending on the
`"type"`. (eg a schema with `"type": "sms"` must have a
`"phone_number"` value and it cannot have an `"email_address"`)
Update the format_checkers to raise the specific exception that why the validator can handle multiple messages.
Which led to a refactor of build_error_message.
