Refactored Optimize(...) function to throw exceptions instead of returning null

Refactored Optimize(...) function to throw exceptions instead of returning null

 * INetwork::Optimize(...) states that the function should throw an exception
   if it fails but the implementation in Network.cpp returned null in some
   scenarios instead. This has led to some confusion amongst users.

Signed-off-by: Mike Kelly <mike.kelly@arm.com>
Change-Id: I358d1293232c9464772aa0e39ab3355e3570c823