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.
