MDL-46148 qtype_calculated: fix validation when importing.

In order to do this in a sane way, I cleaned up a lot of old mess,
inclduing:

1. Previously, qtype_calcuated used ->answeres when importing, and
->answer when saving the form. This was crazy, so I fixed it, and
stripped out the code that made the alternative variable name work.

2. Similarly, it could handle ->answer being either an array, such as
you would get form the HTML editor, or a simple string, which is what
you get form the form. I simplified that too.

3. Finally, I made import use a transaction around saving each
question, so we don't get half questions in the database when an error
occurs.

question/format/webct/format.php

@@ -606,12 +606,9 @@ class qformat_webct extends qformat_default {
                 // Calculated Question.
                 $question = $this->defaultquestion();
                 $question->qtype = 'calculated';
-                $question->answers = array(); // No problem as they go as :FORMULA: from webct.
+                $question->answer = array(); // No problem as they go as :FORMULA: from webct.
                 $question->units = array();
                 $question->dataset = array();
-
-                // To make us pass the end-of-question sanity checks.
-                $question->answer = array('dummy');
                 $question->fraction = array('1.0');
                 $question->feedback = array();
 
@@ -739,7 +736,7 @@ class qformat_webct extends qformat_default {
             if (preg_match('~^:FORMULA:(.*)~i', $line, $webctoptions)) {
                 // Answer for a calculated question.
                 ++$currentchoice;
-                $question->answers[$currentchoice] =
+                $question->answer[$currentchoice] =
                         qformat_webct_convert_formula($webctoptions[1]);
 
                 // Default settings.