builtin/worktree.c: use error_errno()

While at there, improve the error message to say _what_ failed to
remove.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -109,7 +109,7 @@ static void prune_worktrees(void)
 		if (ret < 0 && errno == ENOTDIR)
 			ret = unlink(path.buf);
 		if (ret)
-			error(_("failed to remove: %s"), strerror(errno));
+			error_errno(_("failed to remove '%s'"), path.buf);
 	}
 	closedir(dir);
 	if (!show_only)