[TASK] Enhance install tool core update action

A missing typo3_src isn't necessary a warning,
so the warning is not shown during the first install step anymore.
If one doesn't have a linked source, the 'core update' action is disabled.

Change-Id: Ieea48f404f036edf1d75a3cfe41e6074ac1ad6c5
Resolves: #48999
Releases: master
Reviewed-on: https://review.typo3.org/52265


Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>