Exception symbolication

Automatically symbolicate exceptions that cause test failures. With this patch you get:

```
EXCEPTION CDRSpec some failing test
*** -[__NSArrayI objectAtIndex:]: index 4 beyond bounds for empty array
Call stack:
  ...
  *InvalidArrayAccess.m:13
  *CDRSpecSpec.mm:51
  ...
  *main.m:4
  ...
```

Unless anyone has concerns I'm going to merge this in soon.