When I type "/whois idontexist@collabora.co.uk" into Empathy, here's what happens on the wire:

<!-- sent -->
<iq id="559270952839" to="idontexist@collabora.co.uk" type="get" xmlns="jabber:client">
	<vCard xmlns="vcard-temp"/>
</iq>

<!-- received -->
<iq from="idontexist@collabora.co.uk" id="559270952839" to="will.thompson@collabora.co.uk/Queeg" type="error" xmlns="jabber:client">
	<error type="cancel">
		<item-not-found xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>
	</error>
</iq>

But dbus-monitor shows this:

method call sender=:1.146 -> dest=:1.137 serial=1744 path=/org/freedesktop/Telepathy/Connection/gabble/jabber/will_2ethompson_40collabora_2eco_2euk_2fQueeg; interface=org.freedesktop.Telepathy.Connection.Interface.ContactInfo; member=RequestContactInfo
   uint32 146
method return sender=:1.137 -> dest=:1.146 reply_serial=1744
   array [
   ]

So Gabble is returning success (with no info) rather than a failure. This is irrespective of the error returned by the server. For instance, trying a contact at a non-existent domain:

<!-- sent -->
<iq id="127336127425" to="lol@whut.example.com" type="get" xmlns="jabber:client">
	<vCard xmlns="vcard-temp"/>
</iq>

<!-- received -->
<iq from="lol@whut.example.com" id="127336127425" to="will.thompson@collabora.co.uk/Queeg" type="error" xmlns="jabber:client">
	<error type="cancel">
		<remote-server-not-found xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>
		<text xmlns="urn:ietf:params:xml:ns:xmpp-stanzas">Server-to-server connection failed: DNS resolution failed</text>
	</error>
</iq>