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