Added exception message on general communication exception.

Signed-off-by: Michele Cattafesta <michele.cattafesta@mesta-automation.com>

S7.Net/PLC.cs

@@ -219,10 +219,10 @@ namespace S7.Net
 				LastErrorString = socketException.Message;
 				return null;
 	        }
-            catch
+            catch(Exception exc)
             {
                 LastErrorCode = ErrorCode.WriteData;
-                LastErrorString = "";
+                LastErrorString = exc.Message;
                 return null;
             }
         }