Resolves issue #540

[RomanArzumanyan]

Fixes:

• Resolves Way to distinct EOF and decoding error  #540

New features:

• New enum TaskExecInfo introduced which has more detailed information about task execution.
• PyNvDecoder and PyFFmpegDecoder classes now return tuple. First tuple element is the argument method returned before this PER, second tuple element is of type TaskExecInfo.

Caveats:

• Breaks PyNvDecoder and PyFFmpegDecoder API backward compatibility.

Code example:

nvDec = nvc.PyNvDecoder(input="/path/to/video.mkv", gpu_id=0)
surf, info = nvDec.DecodeSingleSurface()
if not surf.Empty():
    print(info)
    # shall output nvc.TaskExecInfo.SUCCESS