Currently the way flows work in apigee this way - It parses through your default.xml (in proxy) and tries to match your request with one of the flow either through the path-suffix like "/resource1, /resource2" or VERB or any other condition you might have. If it does not find any matching condition, it throws the error like above.
You can add a special flow which will be kicked in if the condition matches none of the valid flows you have. You can add a raisefault policy in that flow and add a custom error response through that flow.