[code] better error handling and show available netdevs on some errors

src/main.rs

@@ -6,6 +6,8 @@ mod sysfs;
 
 use std::{env, thread::sleep};
 
+use sysfs::NetdevError;
+
 use crate::{cfg::Config, measure::LinkSpeed};
 
 fn main() -> eyre::Result<()> {
@@ -19,7 +21,12 @@ fn main() -> eyre::Result<()> {
         (Some(_), Some(na)) => na,
         (Some(nc), None) => nc,
         (None, Some(na)) => na,
-        (None, None) => eyre::bail!("No network device specified"),
+        (None, None) => {
+            eyre::bail!(
+                "No network device specified. Please provide a network device name as an argument or in the config file.\n\n{}",
+                NetdevError::available_netdevs_msg()
+            );
+        }
     };
 
     let link_speed = LinkSpeed::new(&netdev_name)?;