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

Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>
Change-Id: I92a83b61a0c89029cbde0a6cc26599691d171660
diff --git a/src/armnn/Network.cpp b/src/armnn/Network.cpp
index f9115ea..3b7a1cf 100644
--- a/src/armnn/Network.cpp
+++ b/src/armnn/Network.cpp
@@ -415,7 +415,7 @@
                     std::stringstream subgraphMsg;
                     subgraphMsg << "Re-assigning backends to " << failedSubgraph.GetLayers().size()
                                 << " layers inside sub-graph " << count++;
-                    ReportWarning(warningMsg.str(), errMessages);
+                    ReportWarning(subgraphMsg.str(), errMessages);
 
                     OptimizationResult reassignmentResult = AssignBackends(optNetObjPtr,
                                                                            settingsCopy,