Make error messages more prominant in a consistent manner.

edit_stack_form.php

lang/en/qtype_stack.php

locallib.php

questiontestrun.php

renderer.php

stack/input/inputbase.class.php

tests/input_checkbox_test.php

tests/input_dropdown_test.php

tests/input_matrix_test.php

tests/input_numerical_test.php

tests/input_radio_test.php