Converting warning to error, which is consistent with babel env config method

Author: weaverryan

lib/WebpackConfig.js

@@ -410,7 +410,7 @@ class WebpackConfig {
             }
 
             if (this.doesBabelRcFileExist()) {
-                logger.warning('The "callback" argument of configureBabel() will not be used because your app already provides an external Babel configuration (e.g. a ".babelrc" or "babel.config.js" file or "babel" key in "package.json"). Use null as a first argument to remove that warning.');
+                throw new Error('The "callback" argument of configureBabel() will not be used because your app already provides an external Babel configuration (e.g. a ".babelrc" or "babel.config.js" file or "babel" key in "package.json"). Use null as the first argument to remove this error.');
             }
         }