add new block layer error values

The current -Exyz errors are not descriptive enough to allow upper layers like DM and MD to decide how to handle IO errors. In many cases all we get is a -EIO. The following patches introduce block layer error values for use with the bio_endio and end_that_request* functions (end_request keeps uptodate).

This is just as a RFC I am still testing and working on them and am hoping to get some comments. All patches were made against the scsi-misc-2.6 tree.