Clear up error message in defense mechanism against existing tracing.

This package should never have already set it itself prior to this point,
but the circumstance exists that something (anything) else could have
already used the `extensions` sink to attach a value.

This guards against that.

packages/apollo-tracing/src/index.ts

@@ -5,6 +5,8 @@ import {
 } from 'graphql';
 import { ApolloServerPlugin } from "apollo-server-plugin-base";
 
+const { PACKAGE_NAME } = require("../../package.json").name;
+
 export interface TracingFormat {
   version: 1;
   startTime: string;
@@ -88,8 +90,11 @@ export const plugin = (_futureOptions = {}) => (): ApolloServerPlugin => ({
         const extensions =
           response.extensions || (response.extensions = Object.create(null));
 
+        // Be defensive and make sure nothing else (other plugin, etc.) has
+        // already used the `tracing` property on `extensions`.
         if (typeof extensions.tracing !== 'undefined') {
-          throw new Error("The tracing information already existed.");
+          throw new Error(PACKAGE_NAME + ": Could not add `tracing` to " +
+            "`extensions` since `tracing` was unexpectedly already present.");
         }
 
         // Set the extensions.