Logic for registering and checking properties of registrable classes was
broken, allowing to ignore some errors.
Recent fix rBeb798de101a `broke` the result of the
pyapi_idprop_datablock test, because previously that test would fail
(i.e. suceed, as it is an 'expected to break test') for a reason it was
not designed to check.
This is the problem with that kind of tests - you cannot really check
that they are failing on the expected reason(s)...
627a344635