Rewrote service trait join() method to allow thread join handles to return values other than ()

src/rpc.rs

@@ -71,7 +71,8 @@ impl Service for JsonRpcService {
     type JoinReturnType = ();
 
     fn join(self) -> thread::Result<()> {
-        self.thread_hdl.join()
+        self.thread_hdl.join()?;
+        Ok(())
     }
 }