ddm: enable_stats smashes all errors into 500s

[rcgoodfellow]

This code smashes all errors to the enable_stats endpoint into 500s.

maghemite/ddm/src/admin.rs

Lines 382 to 386 in ef0edbb

	.map_err(|e| {
	HttpError::for_internal_error(format!(
	"failed to start stats server: {e}"
	))
	})?,

It would be helpful to be more specific here, in particular if the underlay configuration has not gotten far enough along yet to enable stats, we'll probably want to return something more like a 503.