Packer: `packer fix` should write errors to stderr

[AndiDog]

Using Packer 1.3.1, packer fix Packerfile.in > Packerfile.out would exit 1 without printing anything to the terminal. That is first of all confusing as most users don't have a customized shell which shows good/bad exit code, and second, it appends the error messages to the output file which makes it an invalid Packerfile. Other commands also log errors to stderr.

[SwampDragons]

Thanks for opening; I think this is essentially a duplicate of #6470

[azr]

I believe this should be now solved with this : https://github.com/hashicorp/packer/pull/7466/files#diff-1fca62045a851051294784d1be9ba027R130

[sylviamoss]

Closing this one since is a duplicate and it seems to be fixed by the code @azr shared above. If this is still an issue we can reopen later.

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