Fix the warning message reporting when sub-graphs fail to get optimized
Fix the warning message reporting when sub-graphs fail to get optimized

Signed-off-by: Matteo Martincigh <c6d77cbcee86d63acbf5236b8a439153c50030ae@arm.com>
Change-Id: I92a83b61a0c89029cbde0a6cc26599691d171660

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn