htmx triggers a bunch of different events based on various request life cycle events.

For server errors you'd use htmx:responseError:

https://htmx.org/events/#htmx:responseError

You could write some code like so:

   htmx.on("htmx:responseError", function(evt) {
     GrowlNotification.notify({
       title: 'Request Error',
       description: evt.detail.xhr.responseText
     })
   })

And that would show a growl notification in the event of an request error. Or however you wanted to display it.

I haven't looked into htmx much yet, but am using its predecessor intercooler-js, and this is my code for handling server responses and displaying an error message when it's needed: https://gitlab.com/tildes/tildes/-/blob/master/tildes/static...

I wouldn't consider it particularly elegant, but it's straightforward and has worked well. It uses the "complete.ic" event to trigger whenever an intercooler request finishes, and then uses the response status/code/text to display an appropriate message.