Form tweaker is partly aware of the an error when a mandatory field is missing.
The title on a label in an implementation has information suggesting that form-tweaker is aware of the error.
what is needed is to assign a css class exposing the error to the form-item which created the error.
I think this is the natural way forms behave and that behaviour should arrive to form-tweaker as well.
Overall great module!
keep on the good work.