ASACORE-782 Return consistent error codes for Ping

Remote routing node must return UNKNOWN_NAME if the unique name has
never been requested.

Note that if an app exits, ping to its unique name will return
UNREACHABLE as long as the remote routing node is advertising any
names or there is a session to the remote routing node. Otherwise
the ping will return UNKNOWN_NAME.

Change-Id: I9608302f9443b6a815555b611acc5dab7652db5f
Signed-off-by: Sheshambika Venkateshwaran <sheshamb@qce.qualcomm.com>
Merge "ASACORE-782 Return consistent error codes for Ping" into RB14.12