fix(cli): add more verbose warning if '_debugger' module cannot be found. (#4800)

Author: rodrigc

lib/debugger/debuggerCommons.js

@@ -1,4 +1,18 @@
-var baseDebugger = require('_debugger');
+var baseDebugger;
+try {
+  baseDebugger = require('_debugger');
+} catch (e) {
+  if (e.code == 'MODULE_NOT_FOUND') {
+    console.log('***********************************************************');
+    console.log('* WARNING: _debugger module not available on Node.js 8    *');
+    console.log('* and higher.                                             *');
+    console.log('*                                                         *');
+    console.log('* Use \'debugger\' keyword instead:                       *');
+    console.log('* https://goo.gl/MvWqFh                                   *');
+    console.log('***********************************************************');
+  }
+  throw e;
+}
 var path = require('path');
 
 /**