wal: encapsulate ER_WAL_IO

ER_WAL_IO is set on any WAL error if it was after journal_write()
success. It is not correct, because there can be plenty of
reasons.

In WAL it could be an actual IO error or a cascading rollback in
progress. When used for transactions, it could be an error related
to synchronous transactions like a timeout, or a persistent
ROLLBACK.

These errors are overridden by ER_WAL_IO. The patch encapsulates
the diag installation for bad journal write and for transaction
rollback.

The next patches are going to introduce more error codes and use
proper ones to install a diag.

Part of #6027