If for some reasons loadClass was not working, it was failing totally
silently without any log. If there is any unexpected error in loadClass,
this ended up for example having some tests failing with :

RuntimeError: Malformed property definition <persistent broken
erp5.portal_type.Acquired Property instance '\x00\x00\x00\x00\x00\x00\x12\xb5'>
on /erp5/portal_property_sheets/SimpleItem

While only this was displayed in logs :

ERP5Type/dynamic/lazy_class.py", line 350, in loadClass
    class_definition = generatePortalTypeClass(site, portal_type)
...
AttributeError: __of__

Which was totally unrelated to the real problem.

So a silent initial error was the source of later unrelated errors, making
it very difficult to find the initial source of problem.

So make sure we have logs as soon as we are not able to load a class.