Lint fix missed in pull request

Summary: Just a quick lint fix in `test/frameworks/Isolation.php` that I
missed on approving a pull request

Reviewed By: @fredemmott

Differential Revision: D1735582

Signature: t1:1735582:1418357119:c5c79314f40fb433fc2d6803c7231e3a901efee2
Fix #4380, making get code and get message final

Summary: In order to match with PHP getMessage() and  getCode() changed to final so they can not be overwritten.
Closes https://github.com/facebook/hhvm/pull/4382

Reviewed By: @fredemmott, @JoelMarcey

Differential Revision: D1719077

Signature: t1:1719077:1418362498:f4308198ea42bd097e6816a290f2850a3c5e892c