SerialPortReader: report stream errors

This does not a solution to #4 as it does not handle re-subscribtion
or so, just helps to see what's going on.

Ref: #4

lib/src/reader.dart

@@ -88,7 +88,13 @@ class _SerialPortReaderImpl implements SerialPortReader {
 
   void _startRead() {
     _receiver = ReceivePort();
-    _receiver.listen((data) => _controller.add(data));
+    _receiver.listen((data) {
+      if (data is SerialPortError) {
+        _controller.addError(data);
+      } else {
+        _controller.add(data);
+      }
+    });
     final args = _SerialPortReaderArgs(
       address: _port.address,
       timeout: _timeout,
@@ -119,6 +125,9 @@ class _SerialPortReaderImpl implements SerialPortReader {
           return dylib.sp_nonblocking_read(port, ptr.cast(), bytes);
         });
         args.sendPort.send(data);
+      } else if (bytes < 0) {
+        args.sendPort.send(SerialPortError(
+            SerialPort.lastErrorMessage, SerialPort.lastErrorCode));
       }
     }
     _releaseEvents(events);