KRB5: Print a verbose error message on failure reading the keytab
    
    krb5_kt_resolve() returns 0 when a non-existent keytab is read, which
    means there was no FATAL-level DEBUG message printed to the user in case
    the keytab was missing completely and users had to enable more verbose
    debugging to diagnose failure to start up.
    
    This patch adds both the verbose DEBUG message as well as a syslog
    message.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>