are you referring to specifically returning errors back to a customized form from an action? Or in general returning errors to vRA from deployments of blueprints, xaas, etc.?
Not sure about the former, because I have only ever seen the generic "something is wrong, contact your administrator" type message in a form. I agree that a custom message there would be nice if possible.
But for the latter, I think you can throw what you want back vRA. Although, it does seem to get prepended with a bunch of "extensibility" nonsense that our customers don't care about - would be nice to be able to turn that off.