Rich errors (retry-able vs fatal)

[matt2e]

Use cases:

• Errors that can be handled properly by other modules
• Errors that can result in specific error status codes in gRPC, HTTP, etc
• Erros that define if they can be retried or if there is no use retrying

Design document is here: First-class error types

[matt2e]

@mistermoe has said this is a blocker for using FSM due to all errors currently being retryable