CuTangoSource: add err bool flag and msg in case of DeviceProxy connection error

lib/cutangosource.cpp

@@ -41,5 +41,9 @@ CuData CuTangoSource::read() const {
     else if(dev && d->src.isDbOp()) {
         d->w.db_get(d->src, res);
     }
+    else if(!dev) {
+        res[ttt::Err] = true;
+        res[ttt::Message] = d->dev->getMessage();
+    }
     return res;
 }