[BUGFIX] Uninstall extension with dependency throws Exception

Exception is thrown if uninstalling an extension with
dependecies on that extension. Catch the exception and
show a nice Flashmessage.

Change-Id: I434a4e1e1d83f43a6f5d4c99a82433e35ddd92f9
Resolves: #49722
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22578
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind