Eliminate "retry behavior" enum

[dandavison]

retryable_override: Optional[bool] to capture the user's input, and a computed property err.retryable for the computed output.

err = HandlerError(type=UNAVAILABLE)
err.retryable           # => True
err.retryable_override  # => None

err = HandlerError(type=UNAVAILABLE, retryable_override=False)
err.retryable           # => False
err.retryable_override  # => False