cl-generic: Signal an error when a type specializer won't work

* lisp/emacs-lisp/cl-generic.el (cl--generic--unreachable-types): New var.
(cl-generic-generalizers :extra "typeof"): Use it to signal an
error for those types we can't handle.
2 jobs for master in 113 minutes and 15 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Images
failed #82449
build-image-inotify

01:53:15

 
  Normal
skipped #82450
test-lisp-emacs-lisp-inotify
 
Name Stage Failure
failed
build-image-inotify Build Images
net/http: request canceled (Client.Timeout exceeded while awaiting headers)
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
$ find ${test_name} ! \( -name "*.log" -o -name ${EMACS_TEST_JUNIT_REPORT} \) -type f -delete
find: build-image-inotify-3e96dd4f: No such file or directory
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1