Display problems with payment on the registration page

[webknjaz]

From looking at https://us.pycon.org/2019/registration/register/form/ it's not obvious that there's some problem with the transaction.

I shows "You are registered for PyCon 2019!" there and that's why it took me a while to realize that there was some problem with that.

Ref: https://twitter.com/webKnjaZ/status/1115977046990016513

I guess the best solution would be to show something in red. Like "You are registered but payment has failed. Contact us plz"