Refactor the error handling logic in Storable::_store().

Refactor the error handling logic in Storable::_store().

The logic remains:

1: If the close fails, set $ret to undef, and attempt to unlink the file
2: If the eval set $@, attempt to unlink the file
3: If the unlink is attempted and fails, warn

However, the new code avoids using low-precedence or and a statement modifier,
both for flow control, in the same statement, which I certainly found confusing.
It also now avoids the implicit *three*-state logic for $ret, which previously
was attaching distinct meanings to true, false and undefined.

The new code has (some) flow control inside an if(), and enters a block (which
it seems that the old code strove hard to avoid - blocks are a small runtime
cost). However the block entry is only for the error case, so this should not
matter.