[BUGFIX] Use alerts for new indexed_search BE module

The flash messages needs to be changed to the new classes
which are alerts.

Change-Id: I1520477d10f80191dcf74777522f06757ed6f011
Resolves: #63795
Releases: master
Reviewed-on: http://review.typo3.org/35337
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>