The RPC error raised in server side and got in client side.

# server side
raise GRPC.RPCError, status: :unknown # preferred
raise GRPC.RPCError, status: GRPC.Status.unknown, message: "error message"

# client side
{:error, error} = Your.Stub.unary_call(channel, request)

Error handling can be done with the is_rpc_error/2 guard.

Expanding on the code above, the first option is for the guard to be used in a cond or case, as follows:

cond do
  is_rpc_error(error, GRPC.Status.not_found()) ->
    do_something_when_not_found()

  is_rpc_error(error, GRPC.Status.out_of_range()) ->
    do_something_when_out_of_range()

  true ->
    fallback_code()
end

Another option is for the error handling to be written into a multi-clause function. In such case we must define module attributes for each of the errors we want because the functions in GRPC.Status can't be called directly inside the guard.

...
handle_error(error)
...

@not_found GRPC.Status.not_found()
@out_of_range GRPC.Status.out_of_range()

defp handle_error(error) when is_rpc_error(error, @not_found) do
  # not found
end

defp handle_error(error) when is_rpc_error(error, @out_of_range) do
  # out of range
end

defp handle_error(error) do
  # fallback
end

See GRPC.Status for more details on possible statuses.