bug#25570: 24.5; diff-hunk-kill: Show more clear error messages

[Tino Calancha <tino.calancha@gmail.com>]

emacs -Q:
; Put following content in a buffer in diff-mode:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; START
diff --git a/a b/a
index 184cbf6..d2d9a24 100644
--- a/a
+++ b/a
@@ -1,2 +1,2 @@
-I have a foo in my bar.
+I have a qux in my bar.
  Me too.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; END
< ; Go to beginning of buffer
M-k ; Signal error: Can't find the beginning of the file.
M-K ; Same.
M-n M-k ; Delete hunk OK, but it shows previous error in echo area.

I propose the following patch:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
From 10af5a6466328acff37485364f8332a8043872e8 Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Sun, 29 Jan 2017 20:17:09 +0900
Subject: [PATCH] Ignore error after kill last file or hunk

* lisp/vc/diff-mode.el (diff-hunk-kill): Ignore error after kill last hunk.
(diff-file-kill): Idem.
Go to beginning of hunk before kill (Bug#25570).
---
  lisp/vc/diff-mode.el | 5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index e609ca9f94..3f3ade799b 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -668,7 +668,7 @@ diff-hunk-kill
  	 (inhibit-read-only t))
      (apply 'kill-region bounds)
      (goto-char (car bounds))
-    (diff-beginning-of-hunk t)))
+    (ignore-errors (diff-beginning-of-hunk t))))

  (defun diff-beginning-of-file-and-junk ()
    "Go to the beginning of file-related diff-info.
@@ -720,9 +720,10 @@ diff-beginning-of-file-and-junk
  (defun diff-file-kill ()
    "Kill current file's hunks."
    (interactive)
+  (diff-beginning-of-hunk t)
    (let ((inhibit-read-only t))
      (apply 'kill-region (diff-bounds-of-file)))
-  (diff-beginning-of-hunk t))
+  (ignore-errors (diff-beginning-of-hunk t)))

  (defun diff-kill-junk ()
    "Kill spurious empty diffs."
-- 
2.11.0

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
In GNU Emacs 26.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.6)
  of 2017-01-29
Repository revision: 0073223c23749ffd6bd3f882bc30a82cc37efd2a