* lisp/image/exif.el (exif--direct-ascii-value): New function
(bug#40127).
(exif--parse-directory): Use it to get the correct values for
in-directory (i.e., shorter than 4 octets) strings.
* lisp/image/exif.el (exif-parse-buffer): New function.
(exif-orientation): Ditto.
(exif-error): New error symbol, and adjust all error signalling to
only use that signal.