Allow throwing exception while loading parent class

Allow throwing exception while loading parent class

This is a fix for symfony/symfony#32995.

The behavior is:

* Throwing exception when loading parent/interface is allowed
  (and we will also throw one if the class is simply not found).
* If this happens, the bucket key for the class is reset, so
  it's possibly to try registering the same class again.
* However, if the class has already been used due to a variance
  obligation, the exception is upgraded to a fatal error, as we
  cannot safely unregister the class stub anymore.