Added a hook on SLTest for when exceptions are thrown.

We'd like to display some more debugging information about a test failure when it happens, so we've added a hook to support this.
Per [this comment](https://github.com/inkling/Subliminal/pull/220/files#discussion_r14214147), we could just assign `self.testCaseState = [[SLTestCaseState alloc] init];` here.