feat(debugging): use custom messages when executing scripts to improve stack traces

Now, instead of asynchronous events during executeScript all being described as
`WebDriver.executeScript`, they have their own custom messages. The schedule
shown when debugging will be more informative, as in the example below:

```
-- WebDriver control flow schedule
 |- waiting for debugger to attach
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:37:13)
 |- WebDriver.navigate().to(data:text/html,<html></html>)
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:38:13)
 |- Protractor.get(http://localhost:8081/index.html#/form) - reset url
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:38:13)
 |- Timed out waiting for page to load after 10000ms
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:38:13)
 |- Protractor.get(http://localhost:8081/index.html#/form) - test for angular
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:38:13)
 |- Protractor.get(http://localhost:8081/index.html#/form) - add mock module
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:38:13)
 |- Protractor.get(http://localhost:8081/index.html#/form) - resume bootstrap
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:38:13)
 |- Protractor.waitForAngular()
 |---    at null.<anonymous> (/Users/ralphj/protractor/debugging/failure_spec.js:42:21)
```