nvcodec: improve error reporting on plugin init
nvcodec: Fix reporting of CuDeviceGetCount error
cuda_ret is was always going to be CUDA_SUCCESS in the error log.
nvcodec: Log readable errors when initializing CUDA
It is really difficult for people to figure out why nvcodec has
0 features. Even the debug log is cryptic. Also make sure the errors
go to the ERROR log level, which is more likely to be enabled by
default.