Better error trapping on the options for answer tests.

edit_stack_form.php

lang/en/qtype_stack.php

questiontestedit.php

stack/answertest/at_general_cas.class.php

stack/answertest/atregexp.class.php

stack/answertest/atstring.class.php

stack/answertest/controller.class.php

stack/answertest/numabsolute.class.php

stack/answertest/numrelative.class.php

stack/answertest/stringsloppy.class.php

stack/answertest/tests/at_general_cas_test.php

stack/input/tests/fixtures.class.php

stack/potentialresponsenode.class.php