catch unexpected errors when accessing clslevel attribute

catch unexpected errors when accessing clslevel attribute

Improved the error message that's raised for the case where the
:func:`.association_proxy` construct attempts to access a target attribute
at the class level, and this access fails. The particular use case here is
when proxying to a hybrid attribute that does not include a working
class-level implementation.

Fixes: #7827
Change-Id: Ic6ff9df010f49253e664a1e7c7e16d8546006965