Raise and re-catch NameError from _ModNS

Raise and re-catch NameError from _ModNS

Fixed issue where using a fully qualified path for the classname in
:func:`_orm.relationship` that nonetheless contained an incorrect name for
path tokens that were not the first token, would fail to raise an
informative error and would instead fail randomly at a later step.

Fixes: #7697
Change-Id: I5e1a3aa4c2a6ea5b123be14666f589aec43f4b60