[BUGFIX] Extensionmanager does not show error messages on update failure

The extension manager fails to show any error message if an extension
update fails.

Convert exceptions (e.g. dependency fails, TER download fails) into a
nice flashmessage.
Convert any server errors (uncatchable PHP errors) into a flashmessage.

Resolves: #56823
Releases: 6.2, 6.1
Change-Id: Ife3c6d3dcd23177ba22192dd6ae720352931b538
Reviewed-on: https://review.typo3.org/28738
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Jost Baron
Reviewed-by: Markus Klein
Tested-by: Markus Klein