aaa: Engine should start even if exception occured in load of legacy provider

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1129203
Change-Id: I6ba595253c2569cf581e690b68348e6ae1c804b1
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>