Give helpful feedback when used with an old Babel version.

README.md

@@ -9,10 +9,18 @@
 
 ## Install
 
-```
+For Babel 7.x and grunt-babel v8
+```sh
 $ yarn add --dev grunt-babel @babel/core @babel/preset-env
 ```
 
+For Babel 6.x and grunt-babel v7
+```sh
+$ yarn add --dev grunt-babel@7 @babel-core babel-preset-env
+```
+Note: See the [7.x branch](https://github.com/babel/grunt-babel/tree/7.x) for more examples of
+usage of Babel 6.x. This README is primarily applicable for Babel 7.x
+
 ## Usage
 
 ```js

tasks/babel.js

@@ -1,6 +1,26 @@
 "use strict";
 const path = require("path");
-const babel = require("@babel/core");
+
+let babel;
+try {
+  babel = require("@babel/core");
+} catch (err) {
+  if (err.code === "MODULE_NOT_FOUND") {
+    err.message +=
+      "\n grunt-babel@8 requires Babel 7.x (the package '@babel/core'). " +
+      "If you'd like to use Babel 6.x ('babel-core'), you should install 'grunt-babel@7'.";
+  }
+  throw err;
+}
+
+// Since we've got the reverse bridge package at @babel/core@6.x, give
+// people useful feedback if they try to use it alongside babel-loader.
+if (/^6\./.test(babel.version)) {
+  throw new Error(
+    "\n grunt-babel@8 will not work with the '@babel/core@6' bridge package. " +
+      "If you want to use Babel 6.x, install 'grunt-babel@7'."
+  );
+}
 
 module.exports = function(grunt) {
   grunt.registerMultiTask(