I will tell you what I have done, maybe it would work for you too?
In the same workflow chain that you have your "Display Data - Result of step 1 (Events Register) I have a step with a conditional. In my case, I just use a show “toast” indicator message, that says there is an error.
The conditional on the action to show the toast would be something like:
“When result of step 1 (events-register)'s returned_an_error = yes”
(Make sure you have told Bubble to treat that field as a boolean when you set up your api call).
I then put in the toast message the error status message so a user can see what happened too.