I am using Emacs 26.0.50.  In the cases that I tried, when there is no
comment in the portion of the buffer between the search limit and point,
`comment-search-backward' does not move point to the search limit before
raising an error.  This behaviour contradicts its docstring.  My message
to emacs-devel about this is at
http://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00193.html

Attached are a patch that seems to fix the problem, and a file
containing a test.  Running the test without the change in the patch
fails, and passes with the change:

$ git checkout master

$ emacs -Q -batch -l lisp/newcomment.el -l /tmp/test.el \
  -f ert-run-tests-batch-and-exit
----------------------------------------------------------------------
(ert-test-failed
     ((should
       (=
        (point)
        limit))
      :form
      (= 70 41)
      :value nil))
   FAILED  1/1  test-comment-search-backward-absent
----------------------------------------------------------------------

$ git checkout test-comment-search-forward-absent

$ emacs -Q -batch -l lisp/newcomment.el -l /tmp/test.el \
  -f ert-run-tests-batch-and-exit
----------------------------------------------------------------------
Running 1 tests (2017-09-12 03:31:48+0530)
   passed  1/1  test-comment-search-backward-absent
----------------------------------------------------------------------

Raghu.

--
N. Raghavendra <raghu@hri.res.in>, http://www.retrotexts.net/
Harish-Chandra Research Institute, http://www.hri.res.in/