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

The last parameter is the 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."

See:
https://developer.apple.com/library/ios/documentation/uikit/reference/uikitfunctionreference/index.html#//apple_ref/c/func/UIGraphicsBeginImageContextWithOptions
Merge pull request #509 from hdwr/fix_screenshot_scale

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