txn: single failure point for WAL and TX commit errors

Txn_commit() uses journal_write() function to send requests to WAL
thread. journal_write() can return 0/-1, but these codes have
nothing to do with the actual write result. journal_write() only
signals whether the interaction with WAL thread finished
successfully. It tells nothing about what happened in this
interaction.

To check WAL write result need to look at journal_entry.res field.
As a result, there were 2 failure points to handle. One of them
wasn't handled for synchronous transactions. Not counting 4
failure points in other places:

- When can't prepare the transaction before commit;
- When can't allocate a journal entry;
- When can't append a new entry to the qsync limbo;
- When synchronous transaction completion wait fails.

This patch merges all the failure points into one place for
txn_commit().

Closes #5146