Hard fail for unknown screen size in UIImage+LaunchImage

• UIImage+LaunchImage hard fails (via `assertionFailure()`) in the default case for unknown screen sizes.
	» Has a friendly error message with the current device's screen height and a hint to go update `UIImage+LaunchImage`.
	» Double-checked by commenting out the `case` for a known size.  Works as expected, spitting the error to the log before stopping on EXC_BREAKPOINT or crashing.  Error message shows up for devices in Xcode's Devices organizer window thing as “«datetime» «device name» CardsAgainst[«pid»] <Error>: fatal error: Unable to find a LaunchImage «…»”
	• Moved the pre-existing `launchImageName = "LaunchImage"` to the “won't ever work” comment— it's confusing that this doesn't work and would be all too easy to re-add it in the future ending up with the same mistake.