GPUImageMovie asset and error reporting improvements

Load AVAsset tracks asynchronously even if GPUImageMovie is instantiated with an AVAsset instead of an NSURL.

Enable error reporting from startProcessing by changing it to - (void)startProcessingWithCompletion:(void(^)(BOOL success, NSError *error))completion. Still retains backwards compatibility.

processPlayerItem now starts reading the player item asynchronously. We already expect async in the NSURL case so I hope this is okay.

Also corrected a little typo between AVAssetWriterStatusCompleted and AVAssetReaderStatusCompleted.