Improved Feed Errors and First Feed Entry

We now better check for and report RSS errors so you’ll know if your feed is invalid and why. For example, if the feed isn’t readable, has invalid XML, or is on an invalid hostname, we now say so instead of just reporting the generic error of it being an invalid feed.

For new users, after you add your app name and API key upon logging in, we immediately prompt for the first feed URL making the onboarding process smoother and faster.

Commit #0b3a7a8