emit() does not call handleError on exception.

The [documentation](https://docs.python.org/3.8/library/logging.html#logging.Handler.handleError) for the `handleError` function of `logging.Handler` implies that exceptions should be caught by `emit` and handled by invoking `handleError`. This allows the environment to customer how logging errors are handled.

The fluent logger Handler does not do this: https://github.com/fluent/fluent-logger-python/blob/d1b81bafe442f9cd3059fa07da1fd8de4e92716c/fluent/handler.py#L237

The net result is that exceptions triggered when running the fluent logger (either in format or otherwise) are raised, as opposed to log, which is inconsistent with the rest of the logging Handlers.

The fluent logger should call handleError.

Start in fluent/handler.py around line 237 and compare the Handler's emit behavior with the Python logging.Handler.handleError documentation. Trace exceptions raised during formatting or logging, then verify that the fluent logger invokes handleError instead of propagating them.

索引モデルが issue の本文から書いたものです。