Avoid internal error when user is not Trust admin
    
    When user tries to perform any action requiring communication with
    trusted domain, IPA server tries to retrieve a trust secret on his
    behalf to be able to establish the connection. This happens for
    example during group-add-member command when external user is
    being resolved in the AD.
    
    When user is not member of Trust admins group, the retrieval crashes
    and reports internal error. Catch this exception and rather report
    properly formatted ACIError. Also make sure that this exception is
    properly processed in group-add-member post callback.
    
    https://fedorahosted.org/freeipa/ticket/3390