There are error messages in the HubSpot forms which always appear if, for example, an e-mail address is incomplete or a required field has not been filled in. Currently I cannot change anything about this text. However, it would make sense if we could customize this content.
I'd also see great benefit in being able to edit/customize the error messages in form validation routines. In addition, setting focus to the first affected field would go a long way toward achieving accessibility requirments.