Cancel image processing when capturing still image fails

[mr-v]

Hi!

I'm using FastttCamera to work around nasty iOS SDK crash. However I've found a little issue with FastttCamera itself. STR is similar to one from the radar:

1. Present camera with cameraFlashMode set to FastttCameraFlashModeOn.
2. Take picture.
3. Immediately (before or during the flash), either hit the iPhone's power button to lock it or go to home screen.

In such case taking picture fails and imageDataSampleBuffer is empty. Error's description:

error Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" 
UserInfo={NSUnderlyingError=0x17405ad60 {Error Domain=NSOSStatusErrorDomain Code=-16802 "(null)"},
NSLocalizedFailureReason=An unknown error occurred (-16802), NSLocalizedDescription=The operation could not be completed}

On such errors FastttCamera gets stuck in capturing image state. This fix allows to take another picture after a failure.