getCudaEnabledDeviceCount should return 0 instead of throwing an exception (Feature #1280)

getCudaEnabledDeviceCount should return 0 instead of throwing an exception when no device are found, just like "#if !defined (HAVE_CUDA)".
And maybe getCudaEnabledDeviceCount should return -1 if drivers are too old.
cv::gpu::getCudaEnabledDeviceCount() in OpenCV\modules\gpu\src\initialization.cpp

int cv::gpu::getCudaEnabledDeviceCount()
{
    int count;
    cudaError_t error = cudaGetDeviceCount( &count );
    if (error == cudaErrorNoDevice)
    {
        count = 0;
    }
    else if (error == cudaErrorInsufficientDriver)
    {
        count = -1;
    }
    else //should never happen
    {
        cudaSafeCall(error);
    }
    return count;
}