Default exotic image formats (like .webp) to image-mode

Default exotic image formats (like .webp) to image-mode

* doc/lispref/errors.texi (Standard Errors): Mention the new error.

* lisp/files.el (auto-mode-alist): Add a bunch of image suffixes
to the list (bug#37972) based on the output from "gm convert -list
format" (i.e., graphicsmagick).

* lisp/image-mode.el (image-mode): Rewrite to possibly notify the
user about image-use-external-converter.
(image-mode--setup-mode): Factor out into own function and don't
run under `condition-case' as there's nothing here that should
error.

* lisp/image.el (unknown-image-type): New error.
(image-type): Signal that error so that image-mode can offer
sensible feedback to the user.