Proposal (WebDriverJS) - add a function to executeScript with a custom debugging msg

I was thinking `context` would be like using `WebDriver.call` with a named function; command descriptions wouldn't change, but they'd be sub-tasks (and by extension would be used in error messages). It'd be equivalent to:

    var chrome = require('selenium-webdriver/chrome');
    var assert = require('selenium-webdriver/testing/assert');

    var driver = new chrome.Driver();
    driver.get('http://www.google.com');
    driver.call(function search() {
      driver.findElement({name:'q'}).sendKeys('webdriver');
      driver.findElement({name:'btnG'}).click();
      assert(driver.getTitle()).equalTo('foo');
    });

Which produces:

    CustomError: Google is not equal to foo
        at // ...
    ==== async task ====
    WebDriver.getTitle()
        at // ...
    ==== async task ====
    WebDriver.call(search)
        at // ...