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>