Non-zero exit code for failing to enable an experimental feature flag

With the prior behavior it can be unclear whether the text was a warning
and the feature flag was enabled anyways. We can use a non-zero exit
code and the `{:error, code, text}` return value to make it clear that
the flag wasn't enabled.

Author: the-mikedavis

deps/rabbitmq_cli/lib/rabbitmq/cli/ctl/commands/enable_feature_flag_command.ex

@@ -40,7 +40,7 @@ defmodule RabbitMQ.CLI.Ctl.Commands.EnableFeatureFlagCommand do
     ])} do
           {_, {:badrpc, _} = err} -> err
           {false, :experimental} ->
-              IO.puts("Feature flag #{feature_flag} is experimental. If you understand the risk, use --experimental to enable it.")
+              {:error, RabbitMQ.CLI.Core.ExitCodes.exit_usage(), "Feature flag #{feature_flag} is experimental. If you understand the risk, use --experimental to enable it."}
           _ ->
                 case :rabbit_misc.rpc_call(node_name, :rabbit_feature_flags, :enable, [
                   String.to_atom(feature_flag)