Git bisect run should check for the existence of the script

Hi,

When using 'git bisect run', if the name of the script is misspelled or if the script was not made executable, 'git bisect' considers it to be a failure and stops at the first revision after the one claimed good. It would be better in my eyes to error out.

$ git bisect start
$ git bisect bad HEAD
$ git bisect good HEAD~10
Bisecting: 4 revisions left to test after this (roughly 2 steps)
[344dce312a0cf86d5a5772d54843cc179acaf6e3] bpo-41228: Fix /a/are/ in 
monthcalendar() descripton (GH-21372)
$ git bisect run ./non-existent.sh
running ./non-existent.sh
/usr/lib/git-core/git-bisect: 247: ./non-existent.sh: not found
Bisecting: 2 revisions left to test after this (roughly 1 step)
[6fc732a2116e2c42b0431bb7e2a21719351af755] Fix typo in docs: 'created by 
th' -> 'created by the' (GH-21384)
running ./non-existent.sh
/usr/lib/git-core/git-bisect: 247: ./non-existent.sh: not found
Bisecting: 0 revisions left to test after this (roughly 0 steps)
[8182cc2e68a3c6ea5d5342fed3f1c76b0521fbc1] bpo-39573: Use the Py_TYPE() 
macro (GH-21433)
running ./non-existent.sh
/usr/lib/git-core/git-bisect: 247: ./non-existent.sh: not found
8182cc2e68a3c6ea5d5342fed3f1c76b0521fbc1 is the first bad commit
commit 8182cc2e68a3c6ea5d5342fed3f1c76b0521fbc1
Author: Victor Stinner <vstinner@python.org>
Date:   Fri Jul 10 12:40:38 2020 +0200

     bpo-39573: Use the Py_TYPE() macro (GH-21433)

     Replace obj->ob_type with Py_TYPE(obj).

  Modules/_elementtree.c       | 2 +-
  Objects/abstract.c           | 4 ++--
  Objects/genericaliasobject.c | 2 +-
  Objects/unicodeobject.c      | 4 ++--
  PC/_msi.c                    | 6 +++---
  PC/winreg.c                  | 4 ++--
  Tools/scripts/combinerefs.py | 2 +-
  7 files changed, 12 insertions(+), 12 deletions(-)
bisect run success

Best regards, Jean Abou Samra