fix: `LlamaIndex` callback handler should fail gracefully (#1606)

* Add test that induces an error while handling callbacks

* Add graceful_fallback decorator

* Wire up initial graceful fallbacks

* Fix fallback decorator type hinting

* Patch handler internals instead of the decorated handler itself

* Add documentation

* Improve logging

* Improve testing

* Add graceful fallback tests

* Fix import formatting

* Remove needless typevar definition

* Log tracebacks in `graceful_fallback` decorator

* Remove extra logging from callback handler

* Update llama_index callback handler tests