Modify ShmemInitStruct and ShmemInitHash to throw errors internally,

Modify ShmemInitStruct and ShmemInitHash to throw errors internally,
rather than returning NULL for some-but-not-all failures as they used to.
Remove now-redundant tests for NULL from call sites.

We had to do something about this because many call sites were failing to
check for NULL; and changing it like this seems a lot more useful and
mistake-proof than adding checks to the call sites without them.