style(testability): throw a more informative error when getting testability

The angular.getTestability method requires an element parameter to determine
which Angular application to use. Currently, if the element provided is
undefined or outside of an Angular app, the error message is 'cannot read
property get of undefined'. Improving to a more relevant error message.
Need to add a markdown document in https://github.com/angular/angular.js/tree/master/docs/content/error/ng for this error --- see the other ones for examples