[director] Track failed job updates in event log

This commit fixes a bug that prevented failed instance updates to be
recorded in the event log. Instead of rescuing the error (so event_log
doesn't notice there has been an error and it will record a success update),
now we log the error and raise the exception. In this way, the track method
in event_log will trap the error and it will record it in the event log file.

Also, instead of using a custom logic to check if the job update should be halted
(due to an error, canary or not), use the standard ThreadPool mechanism: if it
detects an exception, it will stop processing new actions.

Refactored also job_updater spec in order to detect all situatuibs and to check
the event log.