ipa-rmkeytab: Check return value of krb5_kt_(start|end)_seq_get
    
    The return value of functions managing the cursor in the keytab
    were not checked or reported in a consistent way. This should
    assure a reasonable error message in case something goes wrong.
    
    https://pagure.io/freeipa/issue/8658
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>