PPP in test_fields.py
test_fields.py: copy field_values before changing them in a test.

I got strange errors when adding a test that did the same
and that had a name that sorted alphabetically before some other tests,
which made those other tests fail.
Changing the test method name to something sorting alphabatically
after test_validation made the test pass...
Return original error during validation when a field already has an error.

This avoids a traceback in Products.CMFFormController.ControllerState
when trying to show all errors on the edit form:

TypeError: 'bool' object has no attribute '__getitem__'