image-dired: Report when a necessary executable is not found

image-dired: Report when a necessary executable is not found

See discussion on:
https://lists.gnu.org/archive/html/emacs-devel/2016-08/msg00552.html
* lisp/image-dired.el (image-dired-cmd-rotate-original-program)
(image-dired-cmd-create-thumbnail-program)
(image-dired-cmd-create-temp-image-program)
(image-dired-cmd-rotate-thumbnail-program)
(image-dired-cmd-write-exif-data-program)
(image-dired-cmd-read-exif-data-program):
Use executable-find to set the defaut value of this option.
(image-dired-cmd-rotate-original-program): Idem.
Search for program 'convert' if 'jpegtran' is not available.
(image-dired-cmd-rotate-original-options):
Set the default value consistent with the executable in
image-dired-cmd-rotate-original-program.
(image-dired-create-thumb, image-dired-display-image)
(image-dired-rotate-thumbnail, image-dired-rotate-original)
(image-dired-set-exif-data, image-dired-get-exif-data):
Throw and error when the executable used in the function is missing.
(image-dired-next-line, image-dired-previous-line):
Use 'forward-line'.