Extract table names when comparing to nrte error

Extract table names when comparing to nrte error

Fixed issue where the process of joining two tables could fail if one of
the tables had an unrelated, unresolvable foreign key constraint which
would raise :class:`_exc.NoReferenceError` within the join process, which
nonetheless could be bypassed to allow the join to complete. The logic
which tested the exception for signficance within the process would make
assumptions about the construct which would fail.

Fixes: #5952
Change-Id: I492dacd082ddcf8abb1310ed447a6ed734595bb7
(cherry picked from commit 498db831718cb5df213b1afdd2027878e0e72fd4)