consistently use log.Check for logging errors that "should not happen", don't influence application flow

sooner or later, someone will notice one of these messages, which will lead us
to a bug.

http/mtasts.go

@@ -60,5 +60,5 @@ func mtastsPolicyHandle(w http.ResponseWriter, r *http.Request) {
 	}
 	w.Header().Set("Content-Type", "text/plain")
 	w.Header().Set("Cache-Control", "no-cache, max-age=0")
-	w.Write([]byte(policy.String()))
+	_, _ = w.Write([]byte(policy.String()))
 }