fixed the cancellation message

[BPC2004]

Closes #3999.

Summary

Changed the cancellation method and made it more clear what happens when you cancel after the deadline.

How to test

1. Make an event and register for it
2. Let the cancellation deadline pass
3. Check the message if it is the same as:

"The deadline has passed, are you sure you want to cancel your registration and pay the estimated full costs of" + str(self.fine) + "?  You will not be able to undo this!"