[$] Improved block-layer error handling

The kernel's filesystem and block layers are places where a lot of thingscan go wrong, often with unpleasant consequences. To make things worse, whenthings do go wrong, informing user space about the problem can be difficultas a consequence of how block I/O works. That can result in user-spaceapplications being unaware of trouble at the I/O level, leading to lost data and enragedusers. There are now two separate (and complementary) proposals underdiscussion that aim to improve how error reporting is handled in the blocklayer.