[TASK] Use flash messages for status of branch export

After a successful or unsuccessful attempt to export a
page branch to a file a short message about the outcome
is displayed. This patch improves the appearance of the
message by using a FlashMessage.

Additionally a call to an undefined method is removed.

Change-Id: I41940ca865211c9347707e731c3b4a90fb11b9ba
Releases: master
Resolves: #69460
Reviewed-on: http://review.typo3.org/42967


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>