util/sss_krb5: debug messages fixes
    
    select_principal_from_keytab() and find_principal_in_keytab()  were changed
    to output more clear error messages.
    
    Resolves: https://pagure.io/SSSD/sssd/issue/4081
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>