Added error handling on -H option

If an unknown host is specified using -H option, output an error
and exit with critical error code.

Change-Id: I4939cf66ba787c6970b7c457f2bdd605118a61c6
Bug-Url: https://bugzilla.redhat.com/918020
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>