MB-8055 Deal with file read errors in mergesort module

Previously read errors were being ignored and a return
value of zero from the read record callback was always
treated as if the end of file was reached, which is not
true because the zero return value could also mean a file
read error happened.

Change-Id: If093bfa17534f62b8d58b9314905e7f037fcb705
Reviewed-on: http://review.couchbase.org/25589
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>