vhost_user_backend: Forward the error from main thread

The main thread returns a Result with any errors from it. Although the
error from the join itself was being returned the real error from the
thread was being ignored so ensure that it is forwarded.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vhost_user_backend/src/lib.rs

@@ -153,9 +153,10 @@ impl<S: VhostUserBackend> VhostUserDaemon<S> {
     /// terminate.
     pub fn wait(&mut self) -> Result<()> {
         if let Some(handle) = self.main_thread.take() {
-            let _ = handle.join().map_err(Error::WaitDaemon)?;
+            handle.join().map_err(Error::WaitDaemon)?
+        } else {
+            Ok(())
         }
-        Ok(())
     }
 
     /// Retrieve the vring worker. This is necessary to perform further