Email API error codes

Branch on error.code.

Code	Status	What to do
`from_required`	400	A new email needs `from`, one of your verified addresses.
`subject_required`	400	A new email needs a `subject`.
`invalid_email`	400	`to` is not an email address.
`sender_not_found`	422	`from` is not one of your email addresses this key can use (GET /email/senders).
`sender_not_verified`	422	The email domain is not verified yet, so nothing can be sent from it.
`recipient_suppressed`	422	An address bounced permanently or reported your email as spam, so nothing can be sent to it.
`recipient_unsubscribed`	422	The address unsubscribed: you can answer their emails, but not start a new thread.
`no_recipient`	422	The email conversation has no customer address to send to.
`attachments_too_large`	400	Attachments may total 10 MB per message.
`api_quota_exceeded`	429	The workspace used its monthly API messages. It resets on the 1st; the plan sets the limit.
`idempotency_key_reused`	422	The Idempotency-Key was already used with a different request.
`idempotency_in_progress`	409	A request with this Idempotency-Key is still being processed. Retry shortly.