[$] Fixing error reporting—again

After a session at last year's LinuxStorage, Filesystem, and Memory Management Summit (LSFMM), Jeff Layton was able tomake some improvements to block-layer errorhandling. Those changes, which added a newerrseq_t type to hold an error number and sequence number, seemedto help and were well received-except by the PostgreSQLdevelopers. So Layton led a session at the 2018 LSFMM to discuss waysto improve things further; it would be followed later in the week with asession by one of the PostgreSQL developers to look at the specifics of theproblem from their perspective.