Fix the resolution of screenshots taken after errors. (320x* => 640x*)

I noticed the screenshots were kind of small and fuzzy (320x568). I swapped out the `UIGraphicsBeginImageContext` function with the one that takes a scale parameter, which defaults to the scale of the device's screen. So the screenshots are now the actual screen size of the device (640x1136).

```objc
void UIGraphicsBeginImageContextWithOptions ( CGSize size, BOOL opaque, CGFloat scale );
```

> The scale factor to apply to the bitmap. If you specify a value of 0.0,
the scale factor is set to the scale factor of the device’s main
screen."

([Apple Docs](https://developer.apple.com/library/ios/documentation/uikit/reference/uikitfunctionreference/index.html#//apple_ref/c/func/UIGraphicsBeginImageContextWithOptions))