[TASK] Show better error messages on failed TER update

This patch shows error messages in a flash message if the ajax request
for a TER-update fails, and resets the extension manager into a sane
state, instead of showing an ever spinning wheel.

Resolves: #46030
Releases: 6.1
Change-Id: I1cc67460a11feb206c53c12a38ad0f6db482e04a
Reviewed-on: https://review.typo3.org/18688
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn