journal: introduce proper error codes

Journal used to have only one error code in journal_entry.res:
-1. It had at least 2 problems:

- There was an assumption that TXN_SIGNATURE_ROLLBACK is the same
  as journal_entry error = -1;

- It wasn't possible to tell if the entry tried to be written and
  failed, or it didn't try yet. Both looked as -1.

The patch introduces a new error code JOURNAL_ENTRY_ERR_UNKNOWN.
The IO error now has its own value: JOURNAL_ENTRY_ERR_IO.

This helps to ensure that a not finished journal entry or a
transaction won't try to obtain a diag error for their result.

Part of #6027