Properly unwind the js stack for generated code

Properly unwind the js stack for generated code

Surround all calls into generated code with a
try {} catch {} statement that resets the jstack
to the correct position.

Like this we properly unwind the js stack in all cases, and
can also use stricter assertions in our ScopedCallData, etc.
classes to check that the stack is healthy.

Change-Id: I7ca03e06ea55007be683305d9c2a6898cf5fc689
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>