Handle non-existent screenname gracefully

main.go

@@ -36,6 +36,14 @@ func probeTwitter(ctx context.Context, target string, registry *prometheus.Regis
 		log.Errorf("decoding json: %s", err)
 		return false
 	}
+	switch l := len(data); {
+	case l == 0:
+		log.Errorf("No data returned. Does screen name exist?")
+		return false
+	case l > 1:
+		log.Errorf("Unexpected (>1) results returned. WTF twitter??")
+		return false
+	}
 	followersGauge.With(prometheus.Labels{"screen_name": target}).Set(float64(data[0].FollowersCount))
 	return true
 }
@@ -60,6 +68,13 @@ func twitterHandler(w http.ResponseWriter, r *http.Request) {
 	h.ServeHTTP(w, r)
 
 }
+
+func bail(err error) {
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
 func main() {
 	var listenAddr string
 	flag.StringVar(&listenAddr, "listen", "0.0.0.0:9700", "Address to listen on")
@@ -69,5 +84,5 @@ func main() {
 		twitterHandler(w, r)
 	})
 	log.Infof("Listening on %s", listenAddr)
-	http.ListenAndServe(listenAddr, nil)
+	bail(http.ListenAndServe(listenAddr, nil))
 }