Ruler is not correctly returning 404 for missing rule groups

[jtlisi]

This line checks for an exact equivalence of a rule group. However, it does not work if the returned error is wrapped.

I tracked the regression back to this PR:

cortex/pkg/ruler/api.go

Lines 429 to 436 in 5501aa6

	if err != nil {
	if err == rulestore.ErrGroupNotFound {
	http.Error(w, err.Error(), http.StatusNotFound)
	return
	}
	http.Error(w, err.Error(), http.StatusBadRequest)
	return
	}

Since this will break behaviour for the cortextool or any rules client that depends on 404 behaviour on the get rule group endpoint, this may be worth fixing in master and 1.8.1.

cc/ @gouthamve @pracucci @pstibrany