[BUG] Re-accessing broken quantity fails

[steven-murray]

Describe the bug
When accessing a quantity errors, using the same object again after updating parameters breaks.

To Reproduce
Steps to reproduce the behavior:

1. Put some error into a quantity
2. Interactively make a framework and try accessing the quantity, giving an error
3. Now, update parameters in the framework.

Expected behavior
Getting quantities should be transactional, so that when it breaks, the object is not forever ruined.