Release the task runner lock before throwing exception

To test this we need to simulate a high concurrency for a 'core_cron' lock. The easiest way to do that is to hack the code.

1. Install testtasks:

   git clone https://github.com/catalyst/moodle-tool_testtasks admin/tool/testtasks

2. Queue a number of tasks:

   php admin/tool/testtasks/cli/queue_adhoc_tasks.php -d=5 -n=50

3. Open lib/classes/task/manager.php in your editor, find the get_next_adhoc_task function and just before the foreach ($records as $record) line add following:

   sleep(20);

4. Open two terminal windows
5. In the first run:

   php admin/cli/cron.php

6. Wait a couple of seconds and then, in the second terminal, run:

   php admin/cli/cron.php

   1. Confirm, that `locktimeout` exception is thrown by the second cron:

      !!! The operation timed out while waiting for a lock. !!!

   2. Confirm, that unreleased lock exception has not been thrown:

      Uncaught coding_exception: Coding error detected, it must be fixed by a programmer: A lock was created but not released at...

7. Run adhoc_task.php from the second terminal:

   php admin/tool/task/cli/adhoc_task.php --execute

   ## Confirm, that `locktimeout` exception is thrown by adhoc_task.php:

   !!! The operation timed out while waiting for a lock. !!!

   1. Confirm, that unreleased lock exception has not been thrown: