fix(gui-client): set IO error type for missing non-tunnel routes (#9777)

On Windows - in order to prevent routing loops - we resolve the best
"non-tunnel" route to a particular host for each IP address. The
resulting source IP is then used as source for packets leaving our
interface. In case the system doesn't have IPv6 connectivity or are
simply no routes available, we fail this "source IP resolver" with an IO
error.

Presently, this uses the "other" IO error type which causes this to be
logged on a WARN level in the event-loop. The IO error types
`HostUnreachable` and `NetworkUnreachable` are expected during normal
operation of Firezone and are therefore only logged on DEBUG.

By changing this IO error type, we fix the WARN log spam on Windows for
machines without IPv6 connectivity.

rust/bin-shared/src/windows.rs

@@ -251,7 +251,10 @@ fn get_best_non_tunnel_route(dst: IpAddr) -> io::Result<Route> {
         .filter(|adapter| is_up(adapter))
         .filter_map(|adapter| find_best_route_for_luid(&adapter.Luid, dst).ok())
         .min()
-        .ok_or(io::Error::other("No route to host"))?;
+        .ok_or(io::Error::new(
+            io::ErrorKind::HostUnreachable,
+            "No route to host",
+        ))?;
 
     tracing::debug!(src = %route.addr, %dst, "Resolved best route outside of tunnel interface");