Hold Call displayed for few seconds after Con. call connected

When two calls are merged to form a Conference call, Secondary info
is displaying the HOLD call in the UI. This was due wrongly handling
the successful call conference state. Fix is to handle failure case only
if call conference request returns with a exception else conference is
successful

Bug: 36497551
Test: Manual
Change-Id: I387eed3ca02af86bd1a32bc3afe0ff348efb546c