Availability API: Better handling of invalid data in GUI

0. Ensure conditional availability (enableavailability) is turned on at system level. 1. Pick an activity to test this on. You will need to know the cmid of the activity. 2. Using database editing software, run this query (modifying it to use your own database prefix, and inserting your cmid number instead of 12345678) to manually ensure the data is invalid for the activity: UPDATE prefix_course_modules SET availability='hahaha' WHERE id=12345678 3. Click 'Purge all caches' link to reset the course cache and use the incorrect value. 4. Look at course page EXPECTED: If you have debugging on, you should see a whole heap of debugging messages 'Error processing availability data', which should mention your activity either by cmid or by name. EXPECTED: Apart from debugging messages, the course page contents should still appear as normal, with the affected activity shown in grey. 5. Edit settings for the activity EXPECTED: Again you will probably see a debugging message. 6. Expand 'Restrict access' fieldset EXPECTED: The controls should be present as normal ('none', with 'Add restriction' button). 7. Save changes and return to website EXPECTED: All the warnings should now go away.