Clarify error when unable to parse listen address

src/main.rs

@@ -1,4 +1,4 @@
-use anyhow::Result;
+use anyhow::{Context, Result};
 use futures::stream::{FuturesUnordered, StreamExt};
 use std::net::ToSocketAddrs;
 use structopt::StructOpt;
@@ -25,7 +25,10 @@ async fn main() -> Result<()> {
         })
     });
 
-    let addrs = args.listen.to_socket_addrs()?;
+    let addrs = args
+        .listen
+        .to_socket_addrs()
+        .context("Couldn't parse the listen address")?;
     let servers = FuturesUnordered::new();
     for addr in addrs {
         let (_, server) = serve(socket_handler).try_bind_ephemeral(addr)?;