Abort payout process if `share_id` is missing

If we have a block with NO `share_id` we abort the entire process. Less
of an issue with proportional since PPS is still being paid out, but a
block round will never actually end.

This will allow for manual intervention by the user. Otherwise blocks
found after the one having issues might trigger the payout process and
pay out shares of the old block in a more recently found one.

Please use #392 for further help on this, I have yet to replicate the
unknown block finder issue with the proper amount of upstream shares.
Merge pull request #394 from TheSerapher/issue-392-abort-payout

Abort payout process if `share_id` is missing