How to show process model error on the interface.

Hi Salonik, 
As far as I know, that's not possible. However, the a!startProecss smart service includes an onError parameter, which you can use to save a custom(eg: couldn't complete the action) error message. This message can be displayed in the user interface to inform users if started process encountered any error.

You could store the error to a process variable, and then display this to the user.