merge-ort: add an err() function similar to one from merge-recursive

Various places in merge-recursive used an err() function when it hit some kind of unrecoverable error. That code was from the reusable bits of merge-recursive.c that we liked, such as merge_3way, writing object files to the object store, reading blobs from the object store, etc. So create a similar function to allow us to port that code over, and use it for when we detect problems returned from collect_merge_info()'s traverse_trees() call, which we will be adding next. While we are at it, also add more documentation for the "clean" field from struct merge_result, particularly since the name suggests a boolean but it is not quite one and this is our first non-boolean usage. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>