notes.h/c: Propagate combine_notes_fn return value to add_note() and beyond

The combine_notes_fn functions uses a non-zero return value to indicate
failure. However, this return value was converted to a call to die()
in note_tree_insert().

Instead, propagate this return value out to add_note(), and return it
from there to enable the caller to handle errors appropriately.

Existing add_note() callers are updated to die() upon failure, thus
preserving the current behaviour. The only exceptions are copy_note()
and notes_cache_put() where we are able to propagate the add_note()
return value instead.

This patch has been improved by the following contributions:
- Jonathan Nieder: Future-proof by always checking add_note() return value
- Jonathan Nieder: Improve clarity of final if-condition in note_tree_insert()

Thanks-to: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes.h

@@ -104,11 +104,13 @@ void init_notes(struct notes_tree *t, const char *notes_ref,
  * note with the empty note (using the given combine_notes function) results
  * in a new/changed note.
  *
+ * Returns zero on success; non-zero means combine_notes failed.
+ *
  * IMPORTANT: The changes made by add_note() to the given notes_tree structure
  * are not persistent until a subsequent call to write_notes_tree() returns
  * zero.
  */
-void add_note(struct notes_tree *t, const unsigned char *object_sha1,
+int add_note(struct notes_tree *t, const unsigned char *object_sha1,
 		const unsigned char *note_sha1, combine_notes_fn combine_notes);
 
 /*
@@ -131,7 +133,10 @@ const unsigned char *get_note(struct notes_tree *t,
 /*
  * Copy a note from one object to another in the given notes_tree.
  *
- * Fails if the to_obj already has a note unless 'force' is true.
+ * Returns 1 if the to_obj already has a note and 'force' is false. Otherwise,
+ * returns non-zero if 'force' is true, but the given combine_notes function
+ * failed to combine from_obj's note with to_obj's existing note.
+ * Returns zero on success.
  *
  * IMPORTANT: The changes made by copy_note() to the given notes_tree structure
  * are not persistent until a subsequent call to write_notes_tree() returns
@@ -139,7 +144,7 @@ const unsigned char *get_note(struct notes_tree *t,
  */
 int copy_note(struct notes_tree *t,
 	      const unsigned char *from_obj, const unsigned char *to_obj,
-	      int force, combine_notes_fn combine_fn);
+	      int force, combine_notes_fn combine_notes);
 
 /*
  * Flags controlling behaviour of for_each_note()