Fix a regression introduced by the yesterday's cleanup.

Fix a regression introduced by the yesterday's cleanup:

recover_wal() can return 0, -1, or 1:
0 - ok, eof read
-1 - error reading wal
1 - can't read eof marker from wal.

recover_remaining_wals() looks at this, and when doing final recovery
deletes wal which don't have eof marker (return value from recover_wal()
== 1).

Yesterday I mistakenly merged 0 and 1 return values into one.
Revert this part of the yesterday's patch.

On the same token, log_io_cursor_next() can't throw an exception,
ever. Take this into account and simplify code.

recover_remaining_wals() continues to stay a mess.