`HttpError::for_status` asserts when passed a 500-level error

[gjcolombo]

for_status takes any HTTP status code as an input, but it unconditionally passes the code to for_client_error, which asserts that the status is 400-level:

dropshot/dropshot/src/error.rs

Lines 204 to 215 in c719b41

	/// Generates an `HttpError` for the given HTTP `status_code` where the
	/// internal and external messages for the error come from the standard label
	/// for this status code (e.g., the message for status code 404 is "Not
	/// Found").
	pub fn for_status(
	error_code: Option<String>,
	status_code: http::StatusCode,
	) -> Self {
	// TODO-polish This should probably be our own message.
	let message = status_code.canonical_reason().unwrap().to_string();
	HttpError::for_client_error(error_code, status_code, message)
	}

dropshot/dropshot/src/error.rs

Lines 143 to 155 in c719b41

	pub fn for_client_error(
	error_code: Option<String>,
	status_code: http::StatusCode,
	message: String,
	) -> Self {
	assert!(status_code.is_client_error());
	HttpError {
	status_code,
	error_code,
	internal_message: message.clone(),
	external_message: message,
	}
	}

I have some error handling code that's tripping this assert: it calls an external service, gets back a Progenitor result, checks to see if the result contains a status, and if so passes that status to HttpError::for_status to re-encode it as an HTTP error. If there's a more idiomatic way to do this specific conversion, that'd be great too, but it seems strange that for_status theoretically allows any status but will explode on a subset of them.