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

[Tino Calancha]

Tino Calancha <tino.calancha@gmail.com> writes:

> 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.
Pushed fix to master branch as commit 01d87bf846b478dea0bfe824678e76089f5af2c7