Similar to what was done in commit aa3f7a10
for FULLTEXT INDEX, we must ensure that MLOG_INDEX_LOAD records will always
be written if redo logging was disabled.

row_merge_build_indexes(): Invoke row_merge_write_redo() also when
online operation is not being executed or an error occurs.
In case of an error, invoke flush_observer->interrupted() so that
the pages will not be flushed but merely evicted from the buffer pool.
Before resuming redo logging, it is crucial for the correctness of
mariabackup and InnoDB crash recovery to flush or evict all affected pages
and to write MLOG_INDEX_LOAD records.