Error out when activeWeight <= 0 in BackoffSweeper (#1702)
Error out when activeWeight <= 0 in BackoffSweeper (#1702)

There is a similar assertion that has been observered to fail once in a
while, but is no longer reproducible. Since this is an exceptional case
we should error our sooner rather than later.

Signed-off-by: Haisheng Yuan <d24d5fc9e8498fe12c79f514db1a69a6d06b8a3c@pivotal.io>
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