Intercept unresolveable comparator attrbute error for attr access

Intercept unresolveable comparator attrbute error for attr access

Fixed bug where a synonym created against a mapped attribute that does not
exist yet, as is the case when it refers to backref before mappers are
configured, would raise recursion errors when trying to test for attributes
on it which ultimately don't exist (as occurs when the classes are run
through Sphinx autodoc), as the unconfigured state of the synonym would put
it into an attribute not found loop.

Fixes: #4767
Change-Id: I9aade8628349fbf538181a0049416cec0a17179c
(cherry picked from commit 234723fa296c9fa7ac63f1c4d877edc7ba120edd)