journal: make journal_write() set diag on error

It used to simply return -1 and set a diag only when OOM happened
inside.

The caller was forced either to ignore the result or set its own
diag regardless of what really happened.

The patch makes journal_write() set a correct diag error when it
returns -1. The only implementation to change was
wal_write_async(). The other implementations always return 0.

Part of #6027