In on_exception pass current exception's instance to on_giveup

[michaeltcoelho]

It would be nice if the current exception being handled in on_exception function could pass the exception's instance to the on_giveup handler.

Also i've noticed that there's only an "empty" raise statement after giving up on retrying. Wouldn't it be correct to reraise the current exception's instance itself?

**When StopIteration is raised the exception being handled when calling the decorated function is reraised.

Snippet with the code i'm talking about:

backoff/backoff/_sync.py

Lines 100 to 108 in 229d30a

	if giveup(e) or max_tries_exceeded or max_time_exceeded:
	_call_handlers(on_giveup, *details)
	raise
	try:
	seconds = _next_wait(wait, jitter, elapsed, max_time_)
	except StopIteration:
	_call_handlers(on_giveup, *details)
	raise e