Stable error model

Fail safely, explain clearly.

Code	Action
`EXPIRED`	Create a new transfer.
`ALREADY_CLAIMED`	Create a new transfer; do not retry the same URL.
`INVALID_STATE`	Do not import; verify schema and validator.
`APP_MISMATCH`	Open in the intended application.
`SCHEMA_MISMATCH`	Use a compatible schema or explicit migration.
`ORIGIN_NOT_ALLOWED`	Add the exact Origin in Dashboard.
`RATE_LIMITED`	Honor Retry-After.
`QUOTA_EXCEEDED`	Inspect UTC-month usage.
`NETWORK_ERROR`	Offer a bounded retry without logging the URL.

See the repository error reference for the complete list.