[BUGFIX] Handle Guzzle exceptions in linkvalidator

* The Guzzle exceptions are handled in order to produce better,
  localized error messages in the Report
* Add a new language key for "Network error"

Resolves: #83611
Releases: master, 8.7
Change-Id: I6480c349cbc3325315f25187df56a4914922e463
Reviewed-on: https://review.typo3.org/55666


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>