MB-9926: correctly propagate errors from find_missing_revs

Previous code was incorrectly adding {error, idrev} pairs to list of
missing revs causing ids that caused get-meta errors to be not
included into list of actually replicated docs

Change-Id: I0c10ab82460d201c95fa65a48479665a5b980af1
Reviewed-on: http://review.couchbase.org/32603
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>