Print an error if creating the tun device fails

It seems that it is not generally understood that network interface
names need to be unique, or that the error generated is "device
busy"/EBUSY in case of a collision. For now, explicitly add an error
log until #213 is implemented.

Signed-off-by: Lee Smet <[email protected]>

mycelium/src/tun/darwin.rs

@@ -87,7 +87,16 @@ pub async fn new(
             "TUN device name must be of the form 'utunXXX...' where X is a digit",
         ))?;
     }
-    let mut tun = create_tun_interface(&tun_config.name)?;
+    let mut tun = match create_tun_interface(&tun_config.name) {
+        Ok(tun) => tun,
+        Err(e) => {
+            error!(
+                "Could not create tun device named \"{}\", make sure the name is not yet in use, and you have sufficient privileges to create a network device",
+                tun_config.name,
+            );
+            return Err(e);
+        }
+    };
     let iface = Iface::by_name(&tun_config.name)?;
     iface.add_address(tun_config.node_subnet, tun_config.route_subnet)?;
 

mycelium/src/tun/linux.rs

@@ -29,7 +29,16 @@ pub async fn new(
     ),
     Box<dyn std::error::Error>,
 > {
-    let tun = create_tun_interface(&tun_config.name)?;
+    let tun = match create_tun_interface(&tun_config.name) {
+        Ok(tun) => tun,
+        Err(e) => {
+            error!(
+                "Could not create tun device named \"{}\", make sure the name is not yet in use, and you have sufficient privileges to create a network device",
+                tun_config.name,
+            );
+            return Err(e);
+        }
+    };
 
     let (conn, handle, _) = rtnetlink::new_connection()?;
     let netlink_task_handle = tokio::spawn(conn);