Fixed exception handling.

Fixed exception handling.

njs_vm_exception() is removed and combined with njs_vm_retval().
vm->exception is removed either, exceptions are now stored in
vm->retval.  It simplifies the client logic, because previously
njs_vm_exception() had to be called if njs_vm_retval() fails.
Additonally, stack traces are now appended to the retval if an exception
happens.