Spec

Monkey-patch RSpec Example Group so that we can track whether an example already failed or not in an after(:each) block

Useful to only capture Selenium screenshots when a test fails

Changed execution_error to be an instance variable (in lieu of a local variable).
Introduced an unique id (example_uid) that is the same for a real Example (passed in after(:each) when screenshot is taken) as well as the corresponding ExampleProxy (passed to the HTML formatter). This unique id gives us a way to correlate file names between generation and reporting time.