Fix traceback of unhandled top-level exceptions

[kusha]

There is no need to raise the exception to log it. Exception log call can receive exc_info parameter.
Before (incorrect traceback):

Traceback (most recent call last):
  File "/home/mbirger/workspace/kafka-logging-handler/kafka_logger/handlers.py", line 194, in unhandled_exception
    raise exception
  File "examples/simple.py", line 63, in <module>
    main()
  File "examples/simple.py", line 59, in main
    raise Exception('No try/except block here')
Exception: No try/except block here

After:

Traceback (most recent call last):
  File "examples/simple.py", line 63, in <module>
    main()
  File "examples/simple.py", line 59, in main
    raise Exception('No try/except block here')
Exception: No try/except block here

[mjahudko]

LGTM +1

[rmonegro]

LGTM