[BUGFIX] Show message if no extension repo exists in extensionmanager

Show a useful error message instead an exception in extensionmanager on
update extension list.

Releases: 7.6, 8.7, master
Resolves: #81146
Change-Id: I9989d17890c381a12f17a0806b46ce269ab1ca5d
Reviewed-on: https://review.typo3.org/53482


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Marc von Schalscha <msa@starfinanz.de>
Tested-by: Marc von Schalscha <msa@starfinanz.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>