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/FeatureChamfer.cpp

@@ -125,6 +125,10 @@ App::DocumentObjectExecReturn *Chamfer::execute(void)
                 return new App::DocumentObjectExecReturn("Resulting shape is invalid");
             }
         }
+        int solidCount = countSolids(shape);
+        if (solidCount > 1) {
+            return new App::DocumentObjectExecReturn("Chamfer: Result has multiple solids. Check parameters.");
+        }
 
         this->Shape.setValue(getSolid(shape));
         return App::DocumentObject::StdReturn;