956dbef krb5: handle KRB5KRB_ERR_GENERIC as unspecific error

Authored and Committed by sbose 6 years ago
    krb5: handle KRB5KRB_ERR_GENERIC as unspecific error
    
    KRB5KRB_ERR_GENERIC is a generic error and we cannot make any
    assumptions about the cause. If there are cases where
    KRB5KRB_ERR_GENERIC is returned and SSSD should behave differently this
    must be solved by other means.
    
    Resolves https://fedorahosted.org/sssd/ticket/2535
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>