A way for question export formats to report errors during export

This is something that got screwed up as part of the conversion to the new file API in 2.0.

In Moodle 1.x we used to write the export file to a temporary folder while outputting a HTML page with warning messages. Then we would trigger the download of the temporary file.

Now, we have to generate the export file as it is being downloaded. Therefore, we have no channel to use for errors.

It is not easy to go back to the old way of doing things while still using the file API. The only option would be to run through the expoort process twice. Once to generat the warnings as part of the question/export.php script and display them; and a second time when the export files is being downloaded. That is not good for performance

So, we need to find some way to communicate problems (like unsupported question types) to the user during or after export, without greatly increasing the amount of processing that is done.