Revert workarounds for $@ broken during 5.13.x - mainly 1f870d5a

Luckily reason prevailed and $@ was left alone during DESTROY time,
the complexity (mnost of it compiled away) is no longer needed

More historic info can be found here: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?p=dbsrgits/DBIx-Class-Historic.git;a=blob_plain;f=useful_guard_objects.html;h=4678884
Some cleanups around loading/use of DBIx::Class::Exception (no func. changes)