image-dired: Signal an error before calling a missing executable

image-dired: Signal an error before calling a missing executable

Reverts commit ca473907
Add a defun to check if an executable exists, and call it
on each function using an external program: when the executable
is not available signal an error.
See discussion on:
https://lists.gnu.org/archive/html/emacs-devel/2016-09/msg00135.html
* lisp/image-dired.el (image-dired--check-executable-exists): New defun.
Throw and error when the executable arg is missing.
(image-dired-display-image, image-dired-rotate-thumbnail)
(image-dired-rotate-original, image-dired-set-exif-data)
(image-dired-get-exif-data):
Use it.