Silence a warning from the test code if Exception::Class isn't installed. Reported by Stefano Ruberti.
Localize $@ to avoid overwriting a previously set $@ while creating a Devel::StackTrace object. This caused a test failure in the Exception::Class tests when run with Perl 5.6.1, but not with 5.8.3. I don't really know how to test for it outside of Exception::Class. Reported by Jesse Erlbaum.