Treat deferred error in deferred:sync!

With this change, `deferred:sync!` does not freeze Emacs when error is
occurred in given deferred chain.  It can be checked by the following
code:

```cl
(deferred:sync!
  (deferred:next
    (lambda () (error "this is uncaught error"))))
```
Merge pull request #15 from tkf/deferred-sync-error

Treat deferred error in deferred:sync!