address klocwork issues (#1750)

doc/examples/update/update.cpp

@@ -46,4 +46,6 @@ int main()
         std::cout << std::get<0>(r);
     }
     std::cout << std::endl;
+
+    return 0;
 }

src/ngraph/placement.cpp

@@ -36,6 +36,7 @@ std::string ngraph::placement_to_string(Placement placement)
     case Placement::GPU: return "GPU";
     case Placement::NNP: return "NNP";
     }
+    throw runtime_error("unhandled placement type");
 }
 
 static Node* take_independent_node_with_placement_priority(

src/ngraph/runtime/cpu/cpu_external_function.cpp

@@ -1358,6 +1358,7 @@ void runtime::cpu::CPU_ExternalFunction::build()
             case CPUTensorRole::CONSTANT: return string("CPUTensorRole::CONSTANT");
             case CPUTensorRole::OUTPUT: return string("CPUTensorRole::OUTPUT");
             }
+            throw runtime_error("unhandled CPU tensor role");
         };
 
         //dump the tensor roles to debug manifest