message: Add better error handling for packetdump example.
diff: @@ -23,6 +23,8 @@ use pnet::packet::ipv6::Ipv6Packet;
  use pnet::packet::tcp::TcpPacket;
  use pnet::packet::udp::UdpPacket;
  use std::env;
  +use std::io::{self, Write};
  +use std::process;
  use std::net::IpAddr;
  fn handle_udp_packet(interface_name: &str, source: IpAddr, destination: IpAddr, packet: &[u8]) {
  @@ -182,7 +184,13 @@ fn handle_packet(interface_name: &str, ethernet: &EthernetPacket) {
  fn main() {
  use pnet::datalink::Channel::Ethernet;
  - let iface_name = env::args().nth(1).unwrap();
  + let iface_name = match env::args().nth(1) {
  + Some(n) => n,
  + None => {
  + writeln!(io::stderr(), "USAGE: packetdump <NETWORK INTERFACE>").unwrap();
  + process::exit(1);
  + },
  + };
  let interface_names_match = |iface: &NetworkInterface| iface.name == iface_name;
  // Find the network interface with the provided name
  