fix(helm): print err if no repositories to update

This prints an error if 'helm update' finds no repositories to update.

cmd/helm/update.go

 package main
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -36,6 +37,10 @@ func runUpdate(cmd *cobra.Command, args []string) error {
 		return err
 	}
 
+	if len(f.Repositories) == 0 {
+		return errors.New("no repositories found. You must add one before updating")
+	}
+
 	updateCharts(f.Repositories, verboseUpdate)
 	return nil
 }