Backwards compatibility mode silently fails to work if there are e.g. unmet dependencies

[36degrees]

Steps to reproduce:

1. Create an app/v6/routes.js file that fails to run correctly (for example, by including an dependency that can't be resolved)
2. Run the kit

Expected result

The kit should surface the error from the routes file.

Actual result

The kit starts, but the v6 routes file is silently ignored.

---

This is because we catch all errors when trying to include the v6/routes.js file, in case it doesn't exist. We silently drop those errors, and just proceed without enabling compatibility mode:

govuk-prototype-kit/server.js

Lines 34 to 39 in f29ce0d

	try {
	v6Routes = require('./app/v6/routes.js')
	useV6 = true
	} catch (e) {
	// No routes.js in app/v6 so we can continue with useV6 false
	}

I think this may be a fairly likely scenario, because when users update the Prototype Kit they overwrite their package.json file, which means any additional dependencies that were installed are lost. They may then try to use the backwards compatibility mode, but it'll fail silently until they resolve their missing dependencies.

[joelanman]

could use fs.existsSync for the check, then require
https://nodejs.org/api/fs.html#fs_fs_existssync_path