suzerain: #3016 more robust find_edge usage

Hope is to avoid picking up near wall artifacts when
what I want is enthalpy behavior far from the wall.
Should degenerate to prior behavior gracefully and
never fail unless it used to fail too.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45038 0bd85f0b-c151-43b9-a1e9-27231104551f