Explicit error on <Route> outside <Router>

When using <Route> outside a valid <Router>, the message is not that explicit. It throws:

`Cannot read property 'route' of undefined`

Even though it's not that bad, I propose we make it even more explicit. Can't be bad to know where the error is coming from. If you made a typo when importing the router, it's not obvious otherwise.

This will throw an error rather than a warning because the library code would crash otherwise.

packages/react-router/modules/Route.js

@@ -33,6 +33,10 @@ class Route extends React.Component {
   }
 
   getChildContext() {
+    if (!this.context.router) {
+      throw new Error('You should not use <Route> or withRoute() outside a valid <Router>');
+    }
+
     return {
       router: {
         ...this.context.router,