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.
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