[SYNCOPE-419] Providing dedicated exception type + audit message in case of user delete failure

git-svn-id: https://svn.apache.org/repos/asf/syncope/branches/1_1_X@1528106 13f79535-47bb-0310-9956-ffa450edef68
[SYNCOPE-419] Merge from 1_1_X

git-svn-id: https://svn.apache.org/repos/asf/syncope/trunk@1528109 13f79535-47bb-0310-9956-ffa450edef68