More descriptive form errors for messaging-destination validation

I'm putting Anonymous Subscriptions on a site. If someone fills an existing account's e-mail address in a new subscription, they get the error
"This destination address is not valid."

I don't think that's a very helpful message so I hacked some possibility to have distinctive error messages for distinctive error conditions.

The problem is that the exact error condition is determined in a function 3 layers away from the form_set_error statement, but that function just returns a boolean value.
To get around that, I hacked an extra byref parameter into various calls.

Do you think that's something useful / the right approach?
(If so, it should probably be set back to CNW because more $error messages need to be set here and there.
If not... well, I won't do further work on it then :) )