wal: refactor wal_write_to_disk()

It didn't have a single fail path. That led to some amount of code
duplication, and it complicated future patches where the journal
entries are going to get a proper error reason instead of default
-1 without any details.

The patch is a preparation for #6027 where it is wanted to have
more detailed errors on journal entry/transaction fail instead
of ER_WAL_IO for everything. Sometimes it can override a real
error like a cascade rollback, or a transaction conflict.

Part of #6027