Ert commands to error if no test at point (bug#25931)

Ert commands to error if no test at point (bug#25931)

* lisp/emacs-lisp/ert.el (ert-results-mode-menu):
Deactivate some items if no test at point.
(ert--results-test-at-point-no-redefinition):
Add option to signal an error rather than return nil.
(ert-results-pop-to-backtrace-for-test-at-point)
(ert-results-pop-to-messages-for-test-at-point)
(ert-results-pop-to-should-forms-for-test-at-point)
(ert-results-describe-test-at-point): Error if no test at point.