Fix #3401 warning on multiple solid

- PartDesign only uses the first result shape
  of an operation and discards the rest without
  warning.

- this also fixes #1707

src/Mod/PartDesign/App/FeatureFillet.cpp

@@ -118,6 +118,11 @@ App::DocumentObjectExecReturn *Fillet::execute(void)
             }
         }
 
+        int solidCount = countSolids(shape);
+        if (solidCount > 1) {
+            return new App::DocumentObjectExecReturn("Fillet: Result has multiple solids. Check parameters.");
+        }
+
         this->Shape.setValue(getSolid(shape));
         return App::DocumentObject::StdReturn;
     }