PartDesign: Fix small bugs in the Pipe feature

- Previously a subtractive pipe on a body with no solids would create a solid. Now Pipe will generate an error.
- Fix incorrect property header. Previously Pipe was identifying itself as Pad (copy paste error)

src/Mod/PartDesign/App/FeaturePipe.cpp

@@ -303,6 +303,9 @@ App::DocumentObjectExecReturn *Pipe::execute(void)
         AddSubShape.setValue(result);
 
         if(base.IsNull()) {
+            if (getAddSubType() == FeatureAddSub::Subtractive)
+                return new App::DocumentObjectExecReturn("Pipe: There is nothing to subtract from\n");
+
             result = refineShapeIfActive(result);
             Shape.setValue(getSolid(result));
             return App::DocumentObject::StdReturn;