Improve warning message, remove dedup check, and unmock console.error

**what is the change?:**
Various changes to get this closer to being finished;
- Improved warning message (thanks @spicyj!!!)
- Removed dedup check on warning
- Remove mocking of 'console.error' in portals test

**why make this change?:**
- warning message improvement: communicates better with users
- Remove dedup check: it wasn't important in this case
- Remove mocking of 'console.error'; we don't want to ignore an
  inaccurate warning, even for an "unstable" feature.

**test plan:**
`yarn test` -> follow-up commits will fix the remaining tests

**issue:**
issue #8854

Author: flarnie

src/renderers/dom/fiber/ReactDOMFiberEntry.js

@@ -531,21 +531,19 @@ function renderSubtreeIntoContainer(
   );
 
   if (__DEV__) {
-    let warned;
-    if (!warned
+    if (
         && container._reactRootContainer
       && container.nodeType !== COMMENT_NODE) {
       const firstChild = container.firstChild;
       const hostInstance =
         DOMRenderer.findHostInstance(container._reactRootContainer.current);
       if (hostInstance) {
-        warned = true;
         warning(
           hostInstance.parentNode === container,
-          'render(...): It looks like the content of this container may have ' +
-          'been updated or cleared outside of React. ' +
-          'This can cause errors or failed updates to the container. ' +
-          'Please call `ReactDOM.render` with your new content.',
+          'render(...): It looks like the React-rendered content of this ' +
+          'container was removed without using React. This is not '
+          'supported and will cause errors. Instead, call ' +
+          'ReactDOM.unmountComponentAtNode to empty a container.',
         );
       }
     }

src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js

@@ -758,13 +758,6 @@ describe('ReactDOMFiber', () => {
     });
 
     it('should update portal context if it changes due to re-render', () => {
-      // We mock out console error because this causes a warning to fire;
-      // it looks as if the content should be in the 'container' div,
-      // and when it's not there after the first render React assumes the
-      // container was cleared outside of React.
-      // for now we are not concerned about this warning firing.
-      spyOn(console, 'error');
-
       var portalContainer = document.createElement('div');
 
       class Component extends React.Component {