PartDesign: Subtractive Loft. Raise error when base is null.

Previously Subtractive loft would create a shape if there is no base
object. This is because the code is shared with Additive Loft, where
that is the right thing to do. Now we check for this, and return error
if there is nothing to subtract from.

src/Mod/PartDesign/App/FeatureLoft.cpp

@@ -179,6 +179,9 @@ App::DocumentObjectExecReturn *Loft::execute(void)
         AddSubShape.setValue(result);
 
         if(base.IsNull()) {
+            if (getAddSubType() == FeatureAddSub::Subtractive)
+                return new App::DocumentObjectExecReturn("Loft: There is nothing to subtract from\n");
+
             Shape.setValue(getSolid(result));
             return App::DocumentObject::StdReturn;
         }