Properly report errors about unbound ERT test symbols.

Properly report errors about unbound ERT test symbols.

Assertions should only be used to check internal consistency within a
package, not to check arguments passed by callers.  Instead, define
and use a new error symbol.

* lisp/emacs-lisp/ert.el (ert-test-unbound): New error symbol.
(ert-select-tests): Use it.

* test/lisp/emacs-lisp/ert-tests.el (ert-test-select-undefined): New
unit test.

* etc/NEWS: Document new behavior.