Log serial read and write errors on Windows.

device/serial/serial_io_handler_win.cc

Index: device/serial/serial_io_handler_win.cc
diff --git a/device/serial/serial_io_handler_win.cc b/device/serial/serial_io_handler_win.cc
index d6bddb273aa9b1757173ba55ae9cdcdf3fcd612a..60a2a332745679ee2c6eba789096ef26b3227a37 100644
--- a/device/serial/serial_io_handler_win.cc
+++ b/device/serial/serial_io_handler_win.cc
@@ -211,6 +211,7 @@ void SerialIoHandlerWin::WriteImpl() {
                         NULL,
                         &write_context_->overlapped);
   if (!ok && GetLastError() != ERROR_IO_PENDING) {
+    VPLOG(1) << "Write failed";
     QueueWriteCompleted(0, serial::SEND_ERROR_SYSTEM_ERROR);
   }
 }
@@ -301,6 +302,7 @@ void SerialIoHandlerWin::OnIOCompleted(
                            NULL,
                            &read_context_->overlapped);
       if (!ok && GetLastError() != ERROR_IO_PENDING) {
+        VPLOG(1) << "Read failed";
         ReadCompleted(0, serial::RECEIVE_ERROR_SYSTEM_ERROR);
       }
     }