Allow JavaScript errors to be raised as exceptions

[odlp]

When configured, upon taking any action:

• The driver checks that no JavaScript errors are present on the page.
• If JavaScript errors are present then an exception is raised.
• The exception includes the JavaScript error messages:

#<Capybara::Webkit::JavaScriptError: {:line_number=>5, :message=>"ReferenceError: Can't find variable: undefinedFunc", :source=>"http://127.0.0.1:56520/"}>

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Motivation

On a project using capybara-webkit the test-suite passed despite a critical JavaScript minification bug. Because the specs didn't defensively assert that error_messages are empty on each navigation, the bug wasn't caught.

I think this option acts as a good safeguard — a strict mode if you will — and will also aid debugging during the TDD cycle. If a failing scenario (now implemented) is still failing, this could speed up figuring out that the failure is due to a different reason than you thought it was.

This feature is inspired by Poltergeist which has an option to raise JS errors as exceptions.

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Questions

• This could also check & raise JavaScript errors on other interactions, like clicking a button. Would this be helpful too? [Update: This PR has now been updated so all interactions perform the check]

[odlp]

Disregarding the Hound single-quoted strings warnings because most of the changes match the current formatting of the files changed, and the thoughtbot Ruby styleguide states:

Prefer double quotes for strings.

We could disable this check by adding a .rubocop.yml entry for this rule like this.

[jferris]

I think this is a good general idea, but I'm not sure where the errors should actually be raised. Raising errors in visit will only catch errors that happen when you first load the page. If the error occurs in a click handler, Ajax request, or something else, it will be raised in the next visit (or not at all).

We may actually want to build this into the command layer between the client and the server, so that the driver will raise an error if there are ever page errors after performing a command.

[odlp]

Ok that's a good direction to head in, I think making the error check apply to all actions makes sense. I'll push this down to the command layer.