[BUGFIX] Scheduler now throws exception on SQL query error

If the query for fetchTask fails for whatever reason, scheduler will
now gracefully throw an exception instead of assuming it had gotten
a valid resource pointer

Resolves: #39085
Releases: master, 6.2
Change-Id: If4d5e0866bf6e5e17fcafbf4287e330366d995fa
Reviewed-on: http://review.typo3.org/36208


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Julian Hofmann <info@julian-hofmann.net>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>