Exposing more details in 500-level Errors

[jgallagher]

From within a dropshot endpoint, we can craft a 500-level error that includes a message that's sent to the client; e.g.,

omicron/wicketd/src/http_entrypoints.rs

Lines 385 to 392 in faa8b8f

	let message =
	format!("Failed to send rack setup request: {err}");
	HttpError {
	status_code: http::StatusCode::SERVICE_UNAVAILABLE,
	error_code: None,
	external_message: message.clone(),
	internal_message: message,
	}

.

However, I don't think we can do the same with the omicron Error type; we have Error::InternalError { .. } and Error::ServiceUnavailable { .. }, but those only provide the internal message, which leaves the client with a generic external message. I imagine this is something we wouldn't use a lot (?), but it came up on #5686 (comment) where it would have been nice to give the operator more information about what they should do to recover.