[ONNX] User friendly assert message when op is not supported (#2478)

* User friendly assert message

* User friendly assert message

* Update UT

src/ngraph/frontend/onnx_import/core/graph.cpp

@@ -123,8 +123,9 @@ namespace ngraph
                 }
             }
 
-            NGRAPH_ASSERT(unknown_operators.empty()) << "unknown operations: "
-                                                     << detail::to_string(unknown_operators);
+            NGRAPH_ASSERT(unknown_operators.empty())
+                << "nGraph does not support the following ONNX operations: "
+                << detail::to_string(unknown_operators);
 
             // Process ONNX graph nodes, convert to nGraph nodes
             for (const auto& node_proto : m_graph_proto->node())

test/onnx_import.in.cpp

@@ -1326,7 +1326,7 @@ TEST(onnx_${BACKEND_NAME}, model_unsupported_op)
     catch (ngraph::ngraph_error const& err)
     {
         std::string what{err.what()};
-        EXPECT_NE(what.find("unknown operations"), std::string::npos);
+        EXPECT_NE(what.find("nGraph does not support"), std::string::npos);
         EXPECT_NE(what.find("FakeOpName"), std::string::npos);
         EXPECT_NE(what.find("AnotherFakeOpName"), std::string::npos);
     }