allow release to run with available erts version

[tsloughter]

I could try adding some sort of major version check, but really feels like just letting the user control their doom is good enough.

I also don't like that the warning will print both when you start the release and when you run any other command, like rpc, ping, etc.. that would make this possibly not useful to some if they depend on that output being a single line result.

[ferd]

If you send the diagnostics line output to stderr you won't break scripts with the results.

[tsloughter]

True, I'll switch it to stderr.