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>

merge-ort.h

@@ -7,7 +7,14 @@ struct commit;
 struct tree;
 
 struct merge_result {
-	/* Whether the merge is clean */
+	/*
+	 * Whether the merge is clean; possible values:
+	 *    1: clean
+	 *    0: not clean (merge conflicts)
+	 *   <0: operation aborted prematurely.  (object database
+	 *       unreadable, disk full, etc.)  Worktree may be left in an
+	 *       inconsistent state if operation failed near the end.
+	 */
 	int clean;
 
 	/*