Fixed Dialyzer warnings

[madasebrof]

The dialyzer doesn't like returning ext/1 from a nif:

def add_numbers(_, _), do: exit(:nif_library_not_loaded)

See https://stackoverflow.com/questions/26612625/erlang-nifs-and-dialyzer-warning

Changed all return values to :erlang.nif_error/1

def add_numbers(_, _), do: :erlang.nif_error(:nif_library_not_loaded)