Adjusted StoreIOState::write() API to allow callers detect write errors.

Adjusted StoreIOState::write() API to allow callers detect write errors.

The errors are reported without these changes, but only via callbacks, some of
which may be asynchronous. Callers such as doPages() would keep calling
write() after the failure. It is probably best to quit early instead of
ignoring "post-error" state in StoreIOState::write().

Added StoreIOState::write() API description.