docs(error/modulerr): add additional debugging help

This simple tip can help to diagnose the error.

Closes #12958

docs/content/error/$injector/modulerr.ngdoc

@@ -6,6 +6,9 @@
 This error occurs when a module fails to load due to some exception. The error
 message above should provide additional context.
 
+A common reason why the module fails to load is that you've forgotten to
+include the file with the defined module or that the file couldn't be loaded.
+
 ### Using `ngRoute`
 
 In AngularJS `1.2.0` and later, `ngRoute` has been moved to its own module.
@@ -24,4 +27,4 @@ angular.module('ng').filter('tel', function (){});
 
 Instead create your own module and add it as a dependency to your application's top-level module.
 See [#9692](https://github.com/angular/angular.js/issues/9692) and
-[#7709](https://github.com/angular/angular.js/issues/7709) for more information
+[#7709](https://github.com/angular/angular.js/issues/7709) for more information