pub fn unavailable(message: impl Into<String>) -> Status {
        Status::new(Code::Unavailable, message)
    }
The service is currently unavailable.  This is a most likely a transient
condition and may be corrected by retrying with a back-off.

See litmus test above for deciding between `FailedPrecondition`,
`Aborted`, and `Unavailable`.
tonic/src/status.rs