agent-tui: display additional checks only when primary check fails

Currently the agent-tui displays always the additional checks (nslookup/ping/http get), even when the primary check (pull image) passes. This may cause some confusion to the user, due the fact that the additional checks do not prevent the agent-tui to complete successfully but they are just informative, to allow a better troubleshooting of the issue (so not needed in the positive case).

The additional checks should then be shown only when the primary check fails for any reason.