Exit maintenance script with non-zero status if execute() returns false

Exit maintenance script with non-zero status if execute() returns false

There's a whole lot of shutdown code in doMaintenance.php that is
skipped when you use exit() directly. In HHVM by default, destructors
are not even called.

There are a few cases where maintenance scripts want to do something after
doMaintenance.php returns, so returning null should continue to mean no exit.

Change-Id: I0891e2ee3af7ef2c64c03b70edcf9e281ce1e7ba