@Borisham,
With the option on the runtime pane you can stop the workflow:
for the Alert you can use the message tool from the developer palette:
Hope this helps.
Regards
@messi007 nailed it. You can also double-down on the "stop running on error" from the checkbox with the Message tool "Error - and Stop Passing Records Through This Tool" error type setting.
@messi007 @CharlieS Thanks for you both, that's great.
I just can't find the way to send an email in case of an error, would you be able to explain that in more details ?
Thanks!