Readd test to get 100% coverage.

Author: janiversen

test/sub_transport/test_basic.py

@@ -111,6 +111,10 @@ async def test_eof_received(self, client):
         """Test eof_received."""
         client.eof_received()
 
+    async def test_error_received(self, client):
+        """Test error_received."""
+        client.error_received(Exception("test call"))
+
     async def test_callbacks(self, use_clc):
         """Test callbacks."""
         client = ModbusProtocol(use_clc, False)