bug#34760: 27.0.50; Suboptimal error message for Checkdoc on an empty file

[Philipp Stephani]

1. Create an empty test file:

   touch /tmp/empty.el
   
2. emacs -Q /tmp/empty.el

3. M-x checkdoc

4. Answer the first question with 'n'.

5. Now you get an error message

   checkdoc-file-comments-engine: Search failed: ";;; .* --- .*
   "

This is an internal error because checkdoc-file-comments-engine uses
re-search-forward, but in one instance doesn't set the NOERROR
argument.  It would be better to catch this and report a nicer error (or
no error).



In GNU Emacs 27.0.50 (build 53, x86_64-pc-linux-gnu, GTK+ Version 3.24.2)
 of 2019-03-05
Repository revision: 1c349c62305d432abf0fa2b6e3f5d754fe4cab79
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Debian GNU/Linux buster/sid

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --enable-gcc-warnings=warn-only
 --enable-gtk-deprecation-warnings --without-pop --with-mailutils
 --enable-checking --enable-check-lisp-object-type --with-modules
 'CFLAGS=-O0 -ggdb3''

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY GNUTLS
FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS
LIBSYSTEMD JSON PDUMPER GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec epa epg epg-config gnus-util
rmail rmail-loaddefs time-date mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils phst skeleton derived edmacro
kmacro pcase ffap thingatpt url-parse auth-source cl-seq eieio
eieio-core cl-macs eieio-loaddefs password-cache json map url-vars
subr-x rx gnutls puny seq byte-opt gv bytecomp byte-compile cconv dbus
xml cl-loaddefs cl-lib elec-pair mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 59174 6779)
 (symbols 48 7772 1)
 (strings 32 19984 2119)
 (string-bytes 1 656314)
 (vectors 16 11248)
 (vector-slots 8 146260 6062)
 (floats 8 23 23)
 (intervals 56 211 0)
 (buffers 992 12))

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