add $this to v[ariable] hhvm debugger cmd

Summary: $this does not end up in 'local variables' until referenced.  This diff adds $this to local variables debugger command.

Reviewed By: @jano

Differential Revision: D1624271
SPL object not implementing Traversable is now a recoverable error

Summary: When an argument to a call like `iterator_count()` did not implement `Traversable`, this used to throw a pure `fatal`. Since 5.6, it now throws a `catchable fatal` (i.e. `E_RECOVERABLE_ERROR` in PHP). Let's match.

Reviewed By: @ptarjan

Differential Revision: D1623321